Quote from crazylilkelly
I was wondering how accurate an ultrasound & a doc's assessment of a fetus's age is w/o knowing when the parents conceived? I mean is there a w/in 2 wk accuracy or something? Thanks!
In short, it depends.
Very early in pregnancy, (1st trimester), U/S is quite accurate in dating a pregnancy. The growth pattern of a fetus is predictible in the early days, and that is how the gestational age is determined on U/S.
Beyond 13 weeks or so, U/S becomes a far less reliable tool in determining length of gestation, because other factors begin to influence the growth pattern of the fetus (genetics, diet, mother's health, use of drugs, alcohol, tobacco, etc.)
In my experience in 11 years of OB and NICU, most women are pretty reliable in determining the dates of their pregnancies, based on their personal knowledge of their menstrual cycles and sexual activity. Unfortunately, not all OBs and CNMs give their patients credit for knowing their own bodies and habits and insist on "imposing" due dates that are off the mark. I can understand that in cases where a woman's cycle is irregular, but not otherwise.
Without going into too much personal detail, I KNEW when both of my girls were conceived. With both pregnancies, I was given "different" due dates by my OB. They were within a few days, so there was no point in quibbling, but it can become an issue when a patient is likely to deliver either pre- or post-term.
On a similar note, U/S is unreliable in determining the fetal weight of a term or near-term pregnancy.