[schainks]

Yes, we don't experiment on humans in space right now, so there is no evidence, sure. My (limited) understanding leads me to this conclusion for (I think) two _very good_ reasons:

1. The twisting and folding of the heart tube is highly dependent on gravity and micro-pressures of circulating blood in the embryo. I learned that from Dr. Larry Taber at Wash U in St. Louis. In microgravity, there's a very strong chance the heart forms incorrectly or if it does form correctly, it conditions itself for zero-g life, so it will have reduced pumping strength because it never needs to move blood from toe to head against gravity. So, even if you gestate a kid correctly in microgravity, the transition to an environment _with_ gravity could be extremely stressful on the body or possibly fatal.

2. Other phases of gestation _depend_ on gravity. The "baby dropping" around the beginning of third trimester is important to kick start the body to prepare for birth. The baby presses on the cervix to stimulate dilation during the process of birth, etc.