Number of humans that have ever lived

Estimates of the number of human beings who have ever lived on Earth constitute an extremely large range, with low estimates around 45 billion, and the highest estimates topping out around 125 billion. Many of the more robust estimates fall into the range of 90 to 110 billion humans.
It is impossible to make anything close to a precise count of the number of human beings who have ever lived, for the following reasons:

· The specific range of characteristics, physiological, psychological and cultural, which define a modern human being, continue to be a subject of intense scholarly research and debate. Until such debates are completely resolved, it is impossible to know just when in human evolutionary history one might begin the count. Resolving these debates would require drawing a thin line between early humans and pre-humans and in the lack of anthropological evidence, the placing of such a line is shaped by the personal interpretation of experts and remains arbitrary at best.

· Even if the scientific community reached wide consensus regarding what characteristics defined the very first human beings, it would be nearly impossible to pinpoint the exact decade, century, or millennium when they first appeared. The fossil record is simply too scarce. Only a few thousand fossils of early humans have ever been found, most no bigger than a tooth or a knuckle bone. While that may sound like a large number, it is truly minuscule when you consider that these few thousand bone fragments must be used to extrapolate the population distribution of millions of early human beings spread thinly across the face of the Earth.

· Until the late 1700s, exceedingly few nations, kingdoms, or empires had ever performed a census that was considered to be anything more than a rough estimate. In many of these early attempts, the focus was not even on counting people, but merely a subset of the people for purposes of taxation or military service. Even with the advent of agencies like the United States Bureau of the Census, reliable census methods and technologies continue to evolve right into the twenty-first century. Even today, these reliable methods and technologies are not applied uniformly in all parts of the world. In short it has been less than two centuries that we have had anything that remotely resembles the robust statistical data that would be needed to perform a calculation regarding the total number of humans that have ever lived.

Considering the relatively small population in the early phases of human development, the first two factors are likely to be less significant than the third. Any such precise population count offered by any source is simply the numeric result of populations statistics, which necessarily used estimates and rough averages as their basis. While they may, if done astutely, provide us with a remote idea about the number of humans who have ever lived on the Earth, the margin of error should always be regarded as being in the billions, or even the tens of billions of people.
"Guesstimating the number of people ever born... requires selecting population sizes for different points from antiquity to the present and applying assumed birth rates to each period..."[18]
According to 2002 data:[18]

· The number who have ever been born is 106,456,367,669

· The world population in mid-2002 was 6,215,000,000

· The percentage of those ever born who were living in 2002 was 5.8%

The claim often made in various popular sources that more than half the humans ever born are alive today, is thus quite exaggerated.

In many forms of life, from plants to man, increases in size and complexity are accompanied by spectacular changes in form. An apple begins as a blossom. Near the base of the flower is the ovary containing ovules, the precursors of seeds. As the blossom transforms itself into a fruit, the skin of the ovule becomes the hard ovule coat of the pip. At the same time, the ovary and other nearby sections of the flower expand enormously, by cell division, until they become the flesh of the apple.

The petals die and fall away.

The woods decay, the woods decay and fall, The vapours weep their burthen to the ground, Man comes and tills the field and lies beneath, And after many a summer dies the swan. Me only cruel immortality Consumes: I wither slowly in thine arms, Here at the quiet limit of the world, A white-haired shadow roaming like a dream The ever-silent spaces of the East, Far-folded mists, and gleaming halls of morn. Alas! for this gray shadow, once a man - So glorious in his beauty and thy choice, Who madest him thy chosen, that he seemed To his great heart none other than a God! I asked thee, "Give me immortality." Then didst thou grant mine asking with a smile, Like wealthy men who care not how they give. But thy strong Hours indignant worked their wills, And beat me down and marred and wasted me, And though they could not end me, left me maimed To dwell in presence of immortal youth, Immortal age beside immortal youth, And all I was, in ashes.

Equally spectacular are the processes which transform a fertilized egg into a baby, the baby into the child, an adolescent, and finally an adult.

The changes that occur in the womb are, of course, the most extraordinary, and the forces that lie behind them are still only partly understood. What stimulates cells to divide?
What starts cell specialization? How do these blobs of matter learn to differentiate, to have different forms and to perform different functions? What are the forces that impel similar cells to come together to form tissues and organs? What sets the time table for the emergence of structures and functions in the embryo?

[One day such knowledge maybe used to control human growth so that every human child is born healthy, and has the opportunity to achieve his or her maximum potential.]

The key molecules that determine human characteristics are the DNA molecules. These were predicted, as a method of reproduction, by the mathematician J Von Neumann, and later discovered and constructed by Watson and Crick. Each DNA molecule – inside what are called the chromosomes (found in the nucleus of each human cell (and indeed the cells of virtually every living thing) is the complex code that determines the structure and perhaps he genetic fate of each individual living thing. Not all animals or plants reproduce sexually - but humans do.

In each human cell there are 46 chromosomes which divide, prior to fertilization of the male sperm and the female egg or ovum in a process called meiosis. This is "mixing process" and also reduces the total number of chromosomes to 23 in each sperm and in each ovum.

All characteristics that make the vastly complex machine called the human body are waiting in those 23 chromosomes - which will later unite to make a new 46 chromosomes -in what are called genes. Each gene is a set of biological "instructions" which will be later used in the nucleus of the cells.

From the father comes the sperm.

From the mother comes the egg.

Sperm are manufactured in almost astronomical quantity. It has been estimated that from 300 to 500 million are released to meet each egg.

Compared to the sperm the egg is enormous.

Chance determines which sex the child is to be.

Sexual reproduction - a rather tedious and tiresome business - which supposedly involves love etc and a subject about which millions of words have been written - takes place.

The sperm makes its way towards the ovum. Usually only one is fertilised. This unification of sperm ad egg is a vast subject on its own. So let's just take typical example of one egg being fertilised by one sperm.

Later she would wake in the middle of the night - in the dream-nightmare I was fucking another woman. She would be hitting me and cursing me.

Yet when we first met at the PYM meetings - I was like a god to her. In the 70s we walked the streets of Ponsonby, arm in arm. She was 18 when I went here - we were both opposed to the Vietnam War and Apartheid. I suppose I was in love.

After the unification of the nuclei of both the sperm and the egg, there begins the "long sequence of complex events, which, if all goes well, will result nine months later in the birth of a baby.

Victor (my son and first child) was born at St Helens which is or was not far from Western Springs, in those days we lived at number 12 Dickens Street, Ponsonby (Ponsonby was in those days populated many people of diverse cultures but there were many Pacific Islanders living in what were considered to be slums). Victor -my oldest - a son - had (or he and his mother had) along and difficult birth lasting some 12 or more hours - a doctor asked me for permission to operate, or to do a Caesarian , and if I would sign a form to that effect - which I did - (I don't know why I was asked - those were "chauvinist" times - although my wife may have been unconscious by then ) but I signed and then he asked me - he was a kind man - if I needed anything - I said yes - could he lend me $10.00? He gave me the sum. I had no money for food or groceries. I was that time earning only $50 a week a paint factory (Berger Paints in Jellicoe Road , Panmure. I used to get to work via bus.) & $10.00 was quite sum in those days but $50 per week was not a high wage even for 1972. I don't know to this day if there was something very much amiss - more than I knew -as Dr Green was working there. Now since that time - I have read Sandra Coney's account of the terrible misadventures and even "evil" practises of Drs. Green and Boneham and others in those days - in her great (and still very important) book: "The Unfortunate Experiment" - now Green and Boneham were involved in the trial which basically accused them of malpractice and of experimenting on women - this included doing caesarians in some cases when they were not necessary - and also the non or wrongly delayed treatment of cervical cancer - but I refer the reader to that book. At the time I was uneasy about green. But there was nothing substantial, just a feeling.

Along birth of that kind (of such a long time) is potentially quite dangerous as in a number of cases the child is deprived of sufficient oxygen - this can cause damage of varying degrees to the child. At school I got a prize for biology (I got the highest marks - my prize was a copy of R L Stephenson’s "Dr Jekyll and My Hyde" [which I read with great enjoyment]) so I knew most of this. The Caesarian may have saved my wife's, and my son's life. So many things happen in one's life - one cannot know everything.

After the unification of the DNA from the sperm with that of the ovum the egg (ovum) is thus fertilised. From a single fertilised egg to a miniature baby takes eight weeks. Complex mechanisms guard 'the delicate embryo from harm - even from it's own mother...' [The body's "normal" reaction would be to abort the foetus, as it is literally a 'foreign body'.]

The womb where the foetus will grow is in the uterus - that is it will expand in the upper uterus. Every month the wall of the uterus grows a thick, spongy lining, rich in blood vessels. If the egg is not fertilised by a sperm, the egg is sloughed off in menstruation. But if [conception] occurs, the lining remains and grows thicker, readying itself to receive the embryo and supply it with nutrients from the mother's body.

The sperm and egg unite in one of the Fallopian tubes, the ducts that lead from the ovaries to the womb.

After this inception, this beginning, this origin.

[3 - 4] days: the new being (beginning as single cells and dividing constantly so that there are 1 2 4 that is 2ⁿ cells (where n is not allowed - in this non-trivial formulation - to be infinity or error) -

That is in the time given above the new thing moves from the tube to the womb (uterus).

The new thing is called a zygote. The zygote is now spherical and begins to change into a hollow ball - a blastocyst , 'with a tiny protuberance at one spot on its inner wall. Most of this little bump, known as the inner cell mass, is destined to become the embryo proper.

The rest of the blastocyst, the chorion, ‘the outer membrane that surrounds the embryo: the amnion, the fluid filled sac in which it floats, and the placenta, or afterbirth, where the exchange of materials takes place between mother and child.’

‘About a week after conception, the blastocyst begins to burrow its wayinto the lining of the womb…’

The trophoblast begins to grow rapidly (when the inner cell mass lies in contact with the uterine lining) and it - ‘…sends out finger-like extensions, called villi, which also work their way into the wall; simultaneously the outer portion forms the chorion. Other trophoblast cells begin to join with part of the inner cell mass to produce the amnion, the fluid content of which serves to cushion the baby against the shocks or pressure of the outside world.

The developing child is well protected inside the womb. While women of course need to be protected and to take care when pregnant – injury such as kicks and so on and even shocks or trauma – especially say after a month or so – are generally not a cause for abortion of the foetus. A serious issue is the danger to baby from Teratogenic agents in the very early stages of development (such agents can be drugs, smoking, alcohol and other harmful substances.

My mother spoke often of her “foolhardiness” - as when she had been locked out of her home; she had tried to climb in a window when she was pregnant one time - now she suffered a miscarriage. Whether this caused the miscarriage I have no idea – it isn’t necessarily the cause – but whatever – mothers suffer terribly for the deaths of their children or miscarriages and so on. We sons – perhaps more than daughters – tend to not see these things so sensitively as we could have – and time passes and one’s mother dies and – I know that I am eternally damned for my heartlessness toward my mother. I failed my mother.

But in life ultimately we all fail – the agent being time and age – which slowly causes us to decline toward nothing and this: tragedy is the existential truth of what we are. A person of eighty will see no value in having had a “wonderful life” if at 80 that person is suffering terribly and most of the old suffer more and more as they age. I am aging I am nearly 60. There is no hope.

Heroism is futile. Set against eternity all is futile. Love dies.

Is it so terrible?

PERHAPS THERE WAS NO HOPE IN HIM – AND YET HE CONTINUED TO LIVE –

EVEN AFTER AUSCHWITZ AND ALL THE TERRIBLE THINGS –

THERE IS HOPE?

But my book says:

‘ Contrary to popular belief, physical or emotional shock rarely makes a mother lose her child.’

Let us summarise:

Prenatal development

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This article is about prenatal development in humans. For other mammals, see mammalian embryogenesis.

Prenatal development is the process in which an embryo or fetus (or foetus) gestates during pregnancy, from fertilization until birth. Often, the terms fetal development, foetal development, or embryology are used in a similar sense.
After fertilization the embryogenesis starts. In humans, when embryogenesis finishes, by the end of the 10th week of gestational age, the precursors of all the major organs of the body have been created. Therefore, the following period, the fetal period, is described both topically on one hand, i.e. by organ, and strictly chronologically on the other, by a list of major occurrences by weeks of gestational age.

Contents [hide] 1 Fertilization 2 Embryonic period 2.1 Changes by weeks of gestational age 3 Fetal period 3.1 By organ 3.2 Changes by weeks of gestational age 4 References 5 External links

[edit] Fertilization

Main article: Fertilization

A sperm fertilizing an ovum

When semen is deposited in the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes. Fertilization of the ovum (egg cell) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its genetic material (DNA). The resulting combination is called a zygote. The term "conception" refers variably to either fertilization or to formation of the conceptus after uterine implantation, and this terminology is controversial.
Like every cell in the body, the zygote contains all of the genetic information unique to an individual. Half of the genetic information residing in the zygote's nucleus comes from the mother's egg nucleus, and the other half from the nucleus of a single sperm. However, the mitochondrial genetic information of the zygote is in its totality contributed by the mother's egg.

[edit] Embryonic period

Main article: Human embryogenesis

The embryonic period in humans begins at fertilization (2nd week of gestation) and continues until the end of the 10th week of gestation (8th week of development).
The zygote spends the next few days traveling down the Fallopian tube. Meanwhile it divides several times to form a ball of cells called a morula. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst. Up to this point there is no growth in the overall size of the embryo, so each division produces successively smaller cells.
The blastocyst reaches the uterus at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida, a glycoprotein shell, occurs. This is required so that the trophectoderm cells of the blastocyst can come into contact with the luminal epithelial cells of the endometrium. (Contrast this with zona hatching, an event that occurs in vitro by a different mechanism, but with a similar result). It then adheres to the uterine lining and becomes embedded in the endometrial cell layer. This process is also called implantation. In most successful pregnancies, the conceptus implants 8 to 10 days after ovulation (Wilcox et al 1999). The inner cell mass forms the embryo, while the outer cell layers form the membranes and placenta. Together, the embryo and its membranes are referred to as a conceptus, or the "products of conception".
Rapid growth occurs and the embryo's main external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or miscarriage, in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing embryo is also susceptible to toxic exposures, such as:

• Alcohol, certain drugs, and other toxins that cause birth defects, such as Fetal alcohol syndrome
• Infection (such as rubella or cytomegalovirus)
• Radiation from x-rays or radiation therapy
• Nutritional deficiencies such as lack of folate which contributes to spina bifida

Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny."^[1] For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.^[1]

[edit] Changes by weeks of gestational age

See also: embryo and Human embryogenesis

A 10mm embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age).

This embryo is also from an ectopic pregnancy, this one in the cornu (the part of the uterus to which the Fallopian tube is attached). The features are consistent with a developmental age of seven weeks (reckoned as the ninth week of pregnancy).

• Week 2 (week of fertilization)
• Fertilization of the ovum to form a zygote which undergoes mitotic cellular division, but does not increase in size. This mitosis is also known as cleavage. A hollow cavity forms marking the blastocyst stage.
• The blastocyst contains only a thin rim of trophoblast cells and a clump of cells at one end known as the "embryonic pole" which include embryonic stem cells.
• The blastocyst hatches from its protein shell (zona pellucida) and implants onto the endometrial lining of the mother's uterus.
• If separation into identical twins occurs, 1/3 of the time it will happen before day 5.^[2]

• Week 3 (1 week following fertilization)
• Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placenta and embryonic membranes.
• Formation of the yolk sac.
• The embryonic cells flatten into a disk, two-cells thick.
• If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoined.
• Primitive streak develops. ^[3]
• Primary stem villi appear. ^[3]

• Week 4 (2 weeks from fertilization - first missed menstrual period)
• A notochord forms in the center of the embryonic disk.
• A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear.
• Heart tubes begin to fuse.
• Gastrulation commences. ^[3]
• Somites, the divisions of the future vertebra, form. ^[3]
• Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. ^[3]

• Week 5 (3 weeks from fertilization)
• The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
• The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primum appear. ^[3]
• Branchial arches, grooves which will form structures of the face and neck, form.
• The neural tube closes.
• The ears begin to form as otic pits.
• Arm buds and a tail are visible.
• Pulmonary primordium, the first traits of the lung appear. ^[3]
• Hepatic plate, the first traits of the liver appear. ^[3]
• Buccopharyngeal membrane ruptures. This is the future mouth. ^[3]
• Cystic diverticulum, which will become the gallbladder, and dorsal pancreatic bud, which will become the pancreas appear. ^[3]
• Urorectal septum begins to form. Thus, the rectal and urinary passageways become separated. ^[3]
• Anterior and posterior horns differentiate in the spinal cord ^[3]
• Spleen appears. ^[3]
• Ureteric buds appear. ^[3]

Embryo at 4 weeks after fertilization.^[4]

• Week 6 (4th week of development)
• The embryo measures 8 mm (1/4 inch) in length.
• Lens pits and optic cups form the start of the developing eye.
• Nasal pits form.
• The brain divides into 5 vesicles, including the early telencephalon.
• Leg buds form and hands form as flat paddles on the arms.
• Rudimentary blood moves through primitive vessels connecting to the yolk sac and chorionic membranes.

• Week 7 (5th week of development)
• The embryo measures 13 mm (1/2 inch) in length.
• Lungs begin to form.
• The brain continues to develop.
• Arms and legs have lengthened with foot and hand areas distinguishable.
• The hands and feet have digits, but may still be webbed.
• The gonadal ridge begins to be perceptible

• Week 8 (6th week of development)
• The embryo measures 18 mm (3/4 inch) in length.
• Nipples and hair follicles begin to form.
• Location of the elbows and toes are visible.
• Spontaneous limb movements may be detected by ultrasound.
• All essential organs have at least begun formation.

[edit] Fetal period

See also: Fetus

The fetal period begins at the end of the 10th week of gestation (8th week of development). Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and by a list of changes by weeks of gestational age.
Because the precursors of the organs are formed, fetus also is not as sensitive to damage from environmental exposures as the embryo. Instead, toxic exposures often cause physiological abnormalities or minor congenital malformation.

[edit] By organ

Each organ has its own development.

• Development of circulatory system
• Heart development
• Development of digestive system
• Tooth development
• Development of endocrine system
• Development of integumentary system
• Development of lymphatic system
• Development of muscular system
• Development of nervous system
• Development of the urinary and reproductive system
• Development of the reproductive system
• Development of the gonads
• Development of the suspensory ligament of the ovary
• Development of respiratory system

[edit] Changes by weeks of gestational age

See also: Fetus

From the 8th week until birth (around 38 weeks), the developing organism is called a fetus. The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation. All major structures are already formed in the fetus, but they continue to grow and develop.

Fetus at 8 weeks after fertilization.^[5]

• Week 9 (7th week of development)
• Embryo measures 30 mm (1.2 inches) in length.
• Intestines rotate.
• Facial features continue to develop.
• the eyelids are more developed.
• the external features of the ear begin to take their final shape.

• Weeks 10 to 13 (8th to 11th week of development)
• The fetus reaches a length of 8 cm (3.2 inches).
• The head comprises nearly half of the fetus' size.
• The face is well formed
• The eyelids close and will not reopen until about the 28th week.
• Tooth buds, which will form the baby teeth, appear.
• The limbs are long and thin.
• The fetus can make a fist with its fingers.
• Genitals appear well differentiated.
• Red blood cells are produced in the liver.

• Weeks 14 to 17 (12th to 15th week of development)
• The fetus reaches a length of about 15 cm (6 inches).
• A fine hair called lanugo develops on the head.
• Fetal skin is almost transparent.
• More muscle tissue and bones have developed, and the bones become harder.
• The fetus makes active movements.
• Sucking motions are made with the mouth.
• Meconium is made in the intestinal tract.
• The liver and pancreas produce fluid secretions.

Fetus at 18 weeks after fertilization.^[6]

• Week 20 (18th week of development)
• The fetus reaches a length of 20 cm (8 inches).
• Lanugo covers the entire body.
• Eyebrows and eyelashes appear.
• Nails appear on fingers and toes.
• The fetus is more active with increased muscle development.
• "Quickening" usually occurs (the mother can feel the fetus moving).
• The fetal heartbeat can be heard with a stethoscope.

• Week 24 (22nd week of development)
• The fetus reaches a length of 28 cm (11.2 inches).
• The fetus weighs about 725 g (1 lb 10 oz).
• Eyebrows and eyelashes are well formed.
• All of the eye components are developed.
• The fetus has a hand and startle reflex.
• Footprints and fingerprints continue forming.
• Alveoli (air sacs) are forming in lungs.

• Week 28 (26th week of development)
• The fetus reaches a length of 38 cm (15 inches).
• The fetus weighs about 1.2 kg (2 lb 11 oz).
• The brain develops rapidly.
• The nervous system develops enough to control some body functions.
• The eyelids open and close.
• The cochleae are now developed, though the myelin sheaths in neural portion of the auditory system will continue to develop until 18 months after birth.
• The respiratory system, while immature, has developed to the point where gas exchange is possible.
• A baby born prematurely at this time may survive, but the possibilities for complications and death remain high.

• Weeks 32 (30th week of development)
• The fetus reaches a length of about 38-43 cm (15-17 inches).
• The fetus weighs about 2 kg (4 lb 6 oz).
• The amount of body fat rapidly increases.
• Rhythmic breathing movements occur, but lungs are not fully mature.
• Thalamic brain connections, which mediate sensory input, form.
• Bones are fully developed, but are still soft and pliable.
• The fetus begins storing iron, calcium, and phosphorus.

• Week 36 (34th week of development)

• The fetus reaches a length of about 40-48 cm (16-19 inches).
• The fetus weighs about 2.5 to 3 kg (5 lb 12 oz to 6 lb 12 oz).
• Lanugo begins to disappear.
• Body fat increases.
• Fingernails reach the end of the fingertips.
• a baby born at 36 weeks has a high chance of survival, but may require medical interventions.

Fetus at 38 weeks after fertilization.^[7]

• Weeks 37 to 40 (35th to 38th week of development)
• The fetus is considered full-term at the 37th week of gestational age.
• It may be 48 to 53 cm (19 to 21 inches) in length.
• The lanugo is gone except on the upper arms and shoulders.
• Fingernails extend beyond fingertips.
• Small breast buds are present on both sexes.
• Head hair is now coarse and thickest.

Human fetus at eight weeks after fertilization.

[edit] References

1. ^ ^{a b} Stephen Jay Gould,. Ontogeny and Phylogeny. Cambridge, Mass: Belknap Press, 206. ISBN 0-674-63941-3.
2. ^ Scott F. Gilbert; with a chapter on plant development by Susan R. Singer (2000). Developmental biology. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-243-7.
3. ^ ^{a b c d e f g h i j k l m n} William J. Larsen (2001). Human embryology. Edinburgh: Churchill Livingstone. ISBN 0-443-06583-7.
4. ^ 3D Pregnancy (large image of fetus at 4 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
5. ^ 3D Pregnancy (large image of fetus at 10 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
6. ^ 3D Pregnancy (large image of fetus at 18 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.
7. ^ 3D Pregnancy (large image of fetus at 38 weeks after fertilization). Retrieved 2007-08-28. A rotatable 3D version of this photo is available here, and a sketch is available here.

• "MedlinePlus Medical Encyclopedia"
• Moore, Keith L. The Developing Human: 3rd Edition. W.B. Saunders Company, Philadelphia PA
• Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. 1999 N Engl J Med. 340(23):1796-9. PMID 10362823
• Ljunger, E, Cnattingius, S, Lundin, C, & Annerén, G. 2005 Chromosomal anomalies in first-trimester miscarriages. Acta Obstetricia et Gynecologica Scandinavica 84(11):1103-1107. PMID 10362823

[edit] External links

Wikimedia Commons has media related to:

Embryology

• Fetal Development Timeline from AboutKidsHealth.ca
• The Changes in Each Stage of Human Development
• Real Time Presentation of Human Embryo Development