Sunday, April 10, 2016

Human Fetal Development - Part 2

Once the blastocyst implants in the uterine wall, it splits into two sets of cells, one of which will become the placenta and one the embryo/fetus.  The placenta sends hormone signals to the woman’s body that it is pregnant.  On Day 14 the zygote stage ends and the embryo stage begins.  This stage is when cell differentiation into different organs begins. The first step, known as grastrulation sees the embryo form three germ layers from which all the tissues will be formed. By Day 20 there is the beginning development of brain, heart, spinal cord and gastro intestinal tract. The embryo is the size of an apple seed or a grain of rice*.

Human fetus at about 4 weeks. Adapted from

Embryo development from 4 weeks to 8 weeks
At the end of 30 days, the embryo is about the size of a raspberry, some 10,000 times larger than the original fertilized egg.  At 8 weeks, the embryo is about 25 mm and weighs 10 grams; by 13 weeks, the end of the first trimester, it is perhaps 80 to 100 mm and 28 grams.

At the end of 26 weeks or the second trimester, the fetus could be born and survive with proper care, developing into a healthy baby.  The third trimester is primarily about the growing and maturing fetus. So far, I have described what we can see and measure in the developing fetus from conception on.  What is not as well-known as it is more difficult to measure is the development of the non-physical.  If birth after 26 weeks is viable then these must be in place by then also.

The bottom line is that the process is extremely complex and it is a wonder that any of us turn out “normal” in the statistical sense of “what usually happens”. (It is also normal ie it usually happens, that one in 800 births is a Down Syndrome baby). So many things from both internal and external sources can affect fetal development.  Alcohol; drugs, licit and illicit. It may not take much and timing is sometimes everything.  If 45 grams of a certain plant are fed twice daily for three to a pregnant sheep beginning on day 29 of pregnancy, the lamb will be stillborn with one huge eye in the middle of its forehead.  If a heifer calf is born twin to a bull, the male hormones over power the female hormones and the female is born sterile.

You can reach your own conclusions to this story but at least now you have a better idea of how things work.

*For a good overview of fetal development from conception through birth, see 


  1. there are two parts to it? My only focus was always the first.
    the Ol'Buzzard

    1. There is a third part when I get it written, that makes the first part possible

  2. It is an amazing process in nearly every species and always raises more questions than answers.

    1. No kidding. Part 3 will deal with some of those questions when I get it written