The picture above is a sonogram (from an ultrasound) at four and one-half months. Although it will still be two and a half months before she has a good chance of living outside the womb, you can see that she has already developed a distinct bone structure, with the head, ribs, hand and leg clearly visible.(She is lying on her back and her head is on the left side of the photo.) The fetal period, the third and final period of prenatal development, begins in the ninth week and lasts until birth. While the fetal period covers the majority of the time of pregnancy, and is when development occurs of what actually comes to look like a baby, the other periods are of crucial importance.
I also have a sonogram from the embryonic period,
which is the second through ninth week of pregnancy. I didn't bother to
include it on this page, but it was of a six-week-old embryo and it looked
like this:
That white dot in the center is the six-week-old embryo. What about the black spots? By the end of the second week, the single, fertilized cell has divided hundreds of times, and these cells begin to separate into two layers, one of which becomes the embryo and the other forms a closed sac around the embryo filled with amniotic fluid.That's what those darker parts are, the amniotic fluid surrounding the embryo.
But
wait, why are there two dark spots there?
I am glad you asked that question. The other spot is the amniotic sac of a second embryo which died. In fact, as much as a third of zygotes die during the germinal period, that is, the first two weeks after fertilization. It is not at all uncommon for women to have been pregnant, or, as in this case, be pregnant with twins, and be unaware of it, because the zygote dies before she even knows that she is pregnant.
Much that is critical in development occurs during the very unexciting and non-photogenic germinal, embryonic and early fetal periods. That little dot does not look nearly as much like a baby as the later sonogram. A sonogram taken at the middle of the eighth month is even more impressive. By this point, the baby weighs over six pounds and her facial features are clearly visible. It's not possible to get a picture of her whole body, because it is much too large to fit on the screen. Ironically, many important developments have taken place before this point, and, often, before the woman even knows she is pregnant.