And the two become one flesh

in more than just poetic meaning. Jimmy Akin points out some fascinating research and comments on the anthropological (and ultimately theological) meanings thereof in Higamus, Hogamus.
I will point out that we midwives have long recognized that the woman at highest risk of certain complications of pregnancy are those women who 1) had the complication in a previous pregnancy and 2) are now pregnant BY A DIFFERENT FATHER!
This also might explain a good bit of why there are more complications of pregnancy in just about every pregnancy begun using assisted reproductive technology (from AI to IVF and all the alphabet soup). It was originally thought that the high complication rate was due to the high rate of multiple gestations - but even the singletons are more likely to have problematic pregnancies.
Natural Law - you can't just ignore it!