What kind of effects will ecstasy have on an unborn child?

MDMA and other amphetamines will easily cross the placenta, so the fetus would certainly be exposed. There don't appear to have been a lot of studies on fetal effects yet - it looks like there isn't evidence of drastic effects, but it still seems like a good thing to avoid. Some studies with pregnant rats do show effects on neurotransmitter levels after birth, which isn't surprising - ecstasy seems to have mid to long term effects on brain chemistry in adults too.

A lot of pills sold as ecstasy contain at least some of something else, so taking it during pregnancy would be a bit of a crapshoot depending on the actual contents. People who take ecstasy also often use other drugs at the same time, like alcohol, cigarettes and cocaine - all of which do have implications for the fetus.

Bottom line, if you or someone else is pregnant and already used ecstasy once, it's probably ok but you'd do better to avoid using it again before delivery. If you haven't used it during pregnancy, definitely stick with that. You pretty much have to err on the side of caution with pregnancy, because you don't know your preexisting risk of complications and no one knows definitely what substances are safe. Keep it natural.