So, a well-educated OB can be a bit casual about the exact age of the fetus because it doesn't really matter all that much.
Remember, being good at computers does not imply being good at medicine. This should be self-evident, but just a quick perusal of some comments here shows the contrary.
I find that in general with medicine my intuition battles against what my intellect knows must be the case. It's not like anything in the body is far away. Cancer isn't even very small. Isn't it weird that it's so hard to treat? Obviously it must be, but it feels like it should be easy. The patient is a foot away. We are inscrutably powerless.
When every day you command computers and they obey, when you can summon distant people and their image appears before you, when you can instantly consult more lore than in the greatest library, when you see people casually toting radiant tablets of spellbinding powers, progress in medicine feels so slow, so analog---so carnal. I know it's wrong, and we live in a world of medical miracles, with vaccines and antibiotics and DNA and everything else, but sometimes I can't help but feel that nothing has happened since Galen. It's not an argument, it's a feeling, but one with pathos that I'm sure many share.