Human Reproduction

A few facts about human reproduction.

Of 100 fertilised zygotes:

• 44 fail to reach 3 months of pregnancy, 7 are lost through induced abortion and 4 lost after the 3rd month. 
• 1.5 die between birth and maturity.
• Of the 43.5 which reach maturity, 10% fail to marry and, of those that do, 15% are infertile.
• Allowing for reproduction in the never married, there are about 8 losses during pregnancy and childbirth.
• So only 35.5% of zygotes successfully reproduce.

Miscarriage rate

• First pregnancy: 5%
• Miscarriage followed by second pregnancy: 24%
• Second pregnancy after successful first pregnancy: 5%

In the past women often died in childbirth, while gorillas and chimps virtually never do.

• A human mother of c. 100lbs (7st 2lbs or 45.4kg) has a 7lb baby while a gorilla mother of c. 200lbs (14st 4lbs or 90.7kg) has a baby of 4lbs.
• Gestation: gorilla 37 weeks, chimpanzee 34 weeks and human 40 weeks.
• Percentage of final brain size at birth: chimpanzee 40.5% and human 23%.
• Reaches 70% of final brain capacity: gorilla at 1 year, chimpanzee at 1 year and human at 3 years.
• Sexual maturity: gorilla at 6 to 7 years, chimpanzee at 9 years and human at 13 years (female).
• Total growth stops: gorilla at 11 years, chimpanzee at 11 years and human at 20 years. 

Suggested that since degree of  helplessness of human infant at birth is so much greater than that of any ape, 'extra-uterine gestation' of 21 months (84 weeks) takes place in humans. Relative to the apes, the human adult departs far less and more slowly from its embryonic form than does the chimpanzee and so remains relatively undifferentiated, facilitated by a retardation in growth.

E. J. Clegg: The Study of Man.
Jared Diamond: The Rise and Fall of the Third Chimpanzee.
Peter J. Wilson: Man: the Promising Primate.