Why Avoid Late-Term Ultrasounds? Part 1

"Your baby is measuring much larger than normal."
"Your baby appears healthy but we are worried about his size; he may not be growing."
"Your amniotic fluid is low".

Generally all of these statements are probably followed by "We'd like to induce you". But you've had a perfectly healthy pregnancy! You've been planning to go into labor on your own. Your baby appears to be happy, how did you get to this point? Should you trust this commonly-used technology to guide decisions about your pregnancy and birth?

Many women find it amazing to see their baby on ultrasound. During my first pregnancy I had 3 ultrasounds, the first at 12 weeks to verify pregnancy, one at 17 weeks for gender, and another at 22 weeks to check anatomy. I was a low-risk, first-time mother. Women with any other risk factors such as gestational diabetes, hypertension, "advanced" maternal age (over 35), suspected genetic abnormalities, or clotting factors may find themselves with many more ultrasounds. As I continued in the pregnancy, however, I became more and more sure that I wanted to avoid an ultrasound late in my pregnancy. Why? Cue the music..

Late. Term. Ultrasounds. What are they good for? Absolutely. NOTHING. (almost)

It seemed to me that ultrasounds in late pregnancy are more often than not used to scare the caregiver and the mother into thinking that something is wrong. It's common knowledge that ultrasound measurements can be, and frequently are, inaccurate. But when it comes down to it...the doctor will usually want to "err on the side of caution".

There have been several studies on ultrasound after 24 weeks gestation in low-risk pregnancies, like this one, a Cochrane review of studies totalling over 25,000 mothers and babies: Read more here. The authors concluded that "routine late pregnancy ultrasound in low risk or unselected populations does not confer benefit on mother or baby. There is a lack of data about the potential psychological effects of routine ultrasound in late pregnancy, and the effects on both short and long term neonatal and childhood outcome."

This study finds that late-term ultrasound estimates of weight are not particularly reliable, and "must be applied with extreme care".

So there are no proven benefits to row-risk pregnant women and their babies, but what about risks?

Risks
A interesting study published in 1993 selected a group of women (sample size 2,834) to either receive "intensive" ultrasounds (5 from 18 weeks to 38 weeks), or "normal", which was only one scan at 18 weeks. There was a statistically significant increase in the intensive group of having a baby with a birthweight below the 10th and 3rd percentile. The irony here is that in my experience, late-term ultrasounds dramatically increase the chances of your baby being diagnosed with IUGR, whether or not the baby's birthweight is actually low at birth. Could the overuse of ultrasound be CAUSING the IUGR? The authors of the study found it "plausible". Perhaps because another study just a few years before had found a similar negative effect on growth and hypothesized that repeated ultrasound exposure most likely affects bone growth, not soft tissue growth. The good news is that a follow-up study found that this difference in growth seemed to disappear by age 8. But the authors of this study recommended that ultrasound use in late pregnancy be restricted to "those women to whom the information is likely to be of clinical benefit." Rather open-ended, but can we find more information on who these women might be?

GreenMedInfo published a blog post entitled "When Do the Benefits of Ultrasound Outweigh the Dangers?" I highly recommend reading the entire article! The author states that the evidence points to several instances where an ultrasound may have some benefit, among them locating the placenta to clear a women to attempt a TOL for a VBAC, when trying to turn a breech baby during an external version, and when utilizing reproductive technology such as IVF. Not included are:

Measuring large for dates
Maternal age greater than 35
Measuring small for dates
To measure the amniotic fluid
Diagnosed Gestational Diabetic
Factor Five Leiden or other clotting abnormalities
Measuring the "thickness" of a uterine scar internally before a woman attempts a VBAC

The author also points out that there are only a handful of issues that an ultrasound might help to diagnose and then treat before the birth of the baby, as well as a high false-positive rate with abnormal ultrasound diagnoses (I will be working on an article on fluid levels soon!), so the possible benefit to most mothers and babies is slim indeed. Even more concerning is that very few large, epidemiological studies have been performed since the allowable acoustic limits for ultrasound machines was increased in 1992. Part of the problem is that finding a control group of unexposed babies would be quite difficult because ultrasound use has become so common!

Although most other studies have been performed on mice, other possible adverse effects of late-term ultrasounds (based on evidence) include:
Damage to the neurological system
Thermally induced teratogenesis (the formation of congenital birth defects)
Left-handedness (in men)
For more studies on the possible adverse effects of routine ultrasound, please see GreenMedInfo's database here.

What is the Bottom Line?

Although ultrasound has been used in the care of pregnant women and babies for several decades now, there isn't much evidence to support it's routine use. There are many animal studies that suggest exposure to ultrasound increases the risk of abnormal neurological development in the brain and for growth restriction of the skeleton. A few women may benefit from ultrasound screening, but the possible risk of repeated exposure should certainly be discussed so that an informed decision can be made. Part 2 will discuss some of the emotional issues that women may face because of the use of ultrasound, and how it may affect them and their care providers throughout the mothers' pregnancies and births.