Accuracy of pregnancy dating scans

When an ultrasound is performed, measurements of the head, abdomen, thigh, and amount of amniotic fluid are done.These measurements are computed automatically in the ultrasound machine’s software.

The simple rule in ultrasound is that when the due date based on ultrasound doesn’t vary from the mother’s dates by more than a week, stick with the mother’s dates; if the ultrasound disagrees by more than a week to ten days, it becomes wiser to rely on the ultrasound.

This doesn’t apply later on in pregnancy for the reasons above. If two ultrasounds one month apart determine coinciding due dates, especially if they agree with the mother’s date based on a last period, the accuracy can be within a couple of days. Babies have their own clock and can come anywhere from three weeks before this exquisitely determined due date till two weeks after.

However, if you have a larger than average baby, the ultrasound will apply the husky figures to the “normal” measurements.

This creates error in that the baby will compute out to be further along than he or she really is.

The computer in the ultrasound (ignorantly) lumps your bigger baby into the dates of babies that big in the “normal” population.

This may then indicate that your 37-week baby is two weeks overdue! Bdend-1g /* Trending Now */ /* Center Rail */ #ya-center-rail .profile-banner-default .ya-ba-title #Stencil . Bgc-lgr #ya-best-answer, #ya-qpage-msg, #ya-question-detail, li.ya-other-answer .tupwrap .comment-text /* Right Rail */ #Stencil . Bxsh-003-prpl #yai-q-answer, #ya-trending, #ya-related-questions h2. Fw-300 .qstn-title #ya-trending-questions-show-more, #ya-related-questions-show-more #ya-trending-questions-more, #ya-related-questions-more /* DMROS */ .So I'm wondering how accurate dating scans/ultrasounds are.Even when the last period is known, ultrasound is reassuring to demonstrate adequate growth, especially when there’s a risk of delayed growth, as in hypertension or smoking, or if there’s the risk of exaggerated growth, as in gestational diabetes.It is not uncommon for babies that are labeled “Large for Gestational Age (LGA)” and “Intra Uterine Growth Restriction (IUGR)” to have monthly or even weekly ultrasounds during the pregnancy.I found out I was pregnant April 8th.my first ultrasound to check to viable pregnancy on May 3rd, they saw the heartbeat and everything was good, told me I was 7wks2days. Then I went a month later to a sonography place after my first midwife appointment.