Ok... if you go by your LMP the date can vary because who knows when *exactly* in your cycle you conceived? They make an educated guess. Some women conceive during their period, you just never know.
With an ultrasound they go by how far the fetus has developed at that point. But again, it can possibly be off, but probably more like a few days rather than a few weeks.
Now, with an ultrasound you are pretty close to figuring out when you conceived but you mist remember that sperm can live in your body anywhere from 3-7 days. The moment you conceive is when the sperm and egg meet, but it could have been sperm from sex 4 days before that. Get it?
FYI that is how it is possible to give birth to non-identical (fraternal? Can't remember the right word lol) twins by 2 different fathers. A little off topic, but an interesting tidbit, I think lol
at 7:37 AM on May. 3, 2011