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 www.invitra.com/two-months-pregnant/
Embryo development from 4 weeks to 8 weeks www.pinterest.com/pin/479633429035019096/
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