How did David know.... (spoilers-inside)

Corporalhiccups
MemberOvomorphJune 04, 20122206 Views12 RepliesHi folks. Long time lurker, first time poster. Having seen the film (and been woefully disappointed) I wanted to ask a question that I haven't seen discussed yet:
How did David know that Shaw's foetus was "about 3 months" old? Fair enough if it had been human he could make that assumption, but seeing as it was not, how could he tell the age?
Furthermore, why did Shaw automatically assume it was something to do with the fact she had been intimate with Holloway the previous day? Couldn't she have become pregnant on Earth three months prior to going into hyper sleep? Couldn't the doctors who told her she was infertile have been wrong? I'm assuming that she had been romantically involved with Holloway prior to the intimacy on the ship.
Any ideas?