Question: how are babies made?

Kate Clancy answered on 15 Jun 2011:

Depends on what you mean, and what kind of babies. I’ll give you the shortest version I can for human babies.

It starts with what Jamie and Emily said — with a fertilized egg. This then travels down the fallopian tube and into the uterus. At this, point, if the fertilized egg doesn’t seem to have anything wrong with it, and the uterus is receptive, the uterus sends a tiny signal to the egg saying “come here! implant yourself here!”

Then with luck the egg hears this and comes and burrows into the lining of the uterus, called the endometrium. It burrows very deep, deeper than in any other species on earth, in order to get the best access it can to maternal energy via her blood.

Then the baby doesn’t do a ton of growing for a while. It does some differentiation of course, but it doesn’t start to get big yet. That’s because lots of energy in that first trimester is used to build the placenta, which is the way the mother and fetus interact. The placenta also protects the mother and fetus from each other, to an extent — the placenta is like a chaperone at a party, to make sure one doesn’t take advantage of the other!

Then the baby grows lots in the second, and then third trimester. In the third trimester the placenta manages to break through even more cell walls, so that it is a single cell layer from the mother. This is the most invasive pregnancy on the planet!

But in time, the fetus begins to realize that it can’t get enough energy through the umbilical cord: it really needs the fat you can get through a nipple from breastmilk if it wants to get any bigger. So it signals to the placenta, and the placenta signals to the mother, and labor begins.

Then, with luck, a baby is born and is able to start breastfeeding right away, in order to stimulate milk production and help bond with his or her mum.