age of fetus

Jolie, BSN · Aug 19, 2006

Hi,
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.

crazylilkelly · Aug 19, 2006

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.

Thanks! I was just wondering b/c my sister is due on 9/22 but the doc thinks they'll have to induce her w/in the next 2 wks. Apparently the baby, Hailey :), is too big. My sister & her husband think she may be further along than the doc thinks. She has an U/S tomorrow so we'll see what happens then. I just can't wait to meet my niece. I get to be in the delivery room!!!

CEG · Aug 20, 2006

Ultrasound is very inaccurate at determining fetal weight at term. I would encourage your sister to question her doctor on the induction- depending on whether the original due date was correct or not, this baby could be very early (as many as 4-6 weeks!) This could cause same dangerous complications for mom and baby! If the dates are wrong and she is due sooner- she will go into labor! Problem solved. Your sister needs to speak with her doctor and do some research before making a decision.

NurseNora, BSN, RN · Aug 22, 2006

If an early U/S is within 2 weeks of a menstrual EDC in a woman with regular periods, the menstrual EDC is the one that should be used.