How much of fetal development is self-led by the fetus (fetal DNA) vs led by the mother. That is, does the mother merely serve as a vessel for growth providing protection, energy, and removing waste for the fetus, or is the mother producing hormones that affect fetal growth and development. The follow-up question is that if the fetus is self-developing, where are the hormones that differentiate males and females coming from since the fetus is presumably not developed enough to synthesize and emit these chemicals themselves. If coming from the mother, how are these hormones emitted in the case of boy/girl fraternal twins (like my own 10 month olds) to ensure proper sexual development of both genders at once? A fairly complicated question... but something I've been curious about since reading that testosterone and estridiol affect the development of more masculine/feminine tendencies in fetuses. Thanks so much!

The small bits of DNA from the sperm and egg is all that is needed. The XX and XY chromosome are set at the moment the sperm enters the egg. Half of your genes come from your father and half come from your mother. They are bundled up as a set of 46 chromosomes. One pair of these, the sex chromosomes, determines what sex you are. An embryo with one X- and one Y-chromosome will be a boy, and one with two X-chromosomes will be a girl. The presence of a Y-chromosome turns on a 'male switch' in the developing embryo. In your case , and my twin mixed sex grandchildren, 2 eggs were fertilized by 2 sperm. The mother does indeed supply nutrients but that is about all. Think about a bird egg, Only DNA and chromosomes, no outside nutrients yet a baby bird develops.
Nothing is accidental, all formed in time and space. Pretty miraculous isn't it?

Hello:

I'm sorry, but fetal development made me think of this: