In early pregnancy, the measurement from the top of the fetal head to the bottom of the pelvis is called the “Crown-Rump” length.Before twelve weeks, it’s difficult to measure much more. In the second trimester, (past the first 12 weeks), the other measurements can be added.The femur (thigh bone) length seems to hold on to it’s accuracy longer than the other parameters, but after 36 weeks it isn’t foolproof.Two ultrasounds taken one month apart that agree with each other on when the due date is, yield a very accurate and reliable answer.Most babies, except in cases of early IUGR and deformities, grow about the same until 20 weeks or so.
Then your baby at 37 weeks will have bigger measurements than babies destined to weigh seven or eight pounds at birth.
The different types of procedures include: Transvaginal Scans – Specially designed probe transducers are used inside the vagina to generate sonogram images.
Most often used during the early stages of pregnancy.
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.
This is used to help assess suspected congenital heart defects.