Ultrasound Guestimates

Be careful when determining the size of your baby by ultrasounds, often used to get an idea of a baby's size. What the practitioner may fail to tell the mother is that third-trimester ultrasounds which attempt to guess the weight of the baby are considered accurate within two pounds (plus or minus). In baby size, that is a huge variation. Because of this I'm always a bit skeptical when someone reports that the technician told them their baby was 6 lbs. 6 oz. (in utero according to ultrasound). This is a calculation based on averages and it would be much more accurate if they would say "your baby is between 4 lbs. 6 oz and 8 lb. 6 oz" to allow for the normal variation - quite a difference! But, that would not sound very scientific and we do like to believe that our high-tech society really can predict this accurately.

It can be a bit risky when critical decisions such as elective inductions or planned c-sections are made based on this kind of information.

For instance, if a woman had a history of delivering a large baby with difficulty, the doctor may advise an early c-section or induction to prevent the baby from "getting too big" but with the two weeks' variation in ultrasound pregnancy dating, there is the potential of delivering a baby that is actually preterm, with accompanying problems. If a planned induction fails because mom's cervix was not actually ready a c-section is likely. So major surgery is another possible risky complication. If the baby is actually a bit premature on top of that (again because of variability of dates), this kind of decision could mean danger for both mother and baby.

Sometimes a baby needs to be delivered early. The goal here is just to be sure you are informed if the only reason you are planning to intervene is because of the ultrasound size measurement. Any baby born by elective induction or c-section more than a week early in the absence of complications that might put the baby or mom in danger to continue the pregnancy to term, should only be done with a lung maturity assessment to be sure the baby is ready. Read what the March of Dimes says about elective inductions and what you should ask your doctor here .

In addition, they have made this statement:

"Clinicians weigh the risk for the mother and the fetus of continuing a medically complicated pregnancy, versus the risks associated with earlier delivery. For some high-risk pregnancies, early delivery may promote better outcomes for both the mother and the baby. The availability of more data on the outcomes of late preterm births will better inform providers and the public about potentially preventable risks. Pregnancies should continue to term if medically and obstetrically advisable, thereby avoiding unnecessary preterm inductions and c-sections. (Source)

Click the post title to see some mothers' experiences with weight guesstimates and the accuracy of them.

The woman's pelvis also has an amazing ability to move and allow delivery, in most cases even of a large baby, so baby and pelvis size do not necessarily mean normal vaginal delivery is impossible. The baby born full-term (ideally 39-40 weeks) has the best chance at good health. It's important not to rush things simply for convenience or avoidance of another few weeks of discomfort.

Want more information about this topic? Click here to learn about the history of ultrasound in obstetrics. And, if you want to see some cool 3-D ultrasound pictures, click here.

Added later - On elective inductions, this is from the Deseret News:

"Some hospitals — including Intermountain Healthcare in Utah — are tightening the rules for elective deliveries because some babies are being delivered too early.

Recent research shows a troubling link between elective inductions and so-called "late preemies" who are at higher risk of breathing disorders and other problems than babies who finish their very last weeks in the womb.

"It was an 'aha' moment for me," recalls Dr. Bryan Oshiro of his visit to a Utah intensive care nursery several years ago, where neonatologists pointed to babies there simply because they'd been induced too soon."