Thanks for the classic images! I found a good summary of the numerous benefits and very small number of risk factors for sex during pregnancy at:
Sex during pregnancy: Safety tips, positions, effects, and information
The only thing I would add is that avoiding risks of STDs is obviously a must.
I think that my first wife actually got hornier during her three pregnancies, even during most of the third trimester. I think it may be instinctual. She just kept right on fucking, both with me and the guy who probably got her pregnant the third time.