Are they accurate? Are they beneficial? Are they harmful?
No test, including ultrasounds, is 100% accurate. They may be beneficial–there are some rare fetal malformations that can be known (or possibly even repaired) prior to birth that will improve neonatal outcomes (for instance, a known heart defect can allow specialists to be at the birth); the position of the baby can be accurately determined, as can the number of babies, etc. There has been little study into the potential harmful effects of ultrasound. Here is an article that discusses this question more fully. Among other things, it talks about the increased rate of left-handedness among boys who were exposed to prenatal ultrasound, and says, “left-handedness is statistically linked to many cognitive and developmental problems ranging from learning difficulties to autism to epilepsy.” Another thing this article discusses, is the fact that most of the studies on ultrasound safety were done in the 70s, while modern ultrasound machines are capable of much higher output; plus women are having more ultrasounds per pregnancy. Some women have ultrasounds every doctor’s visit, and there are even places you can go for “keepsake ultrasounds.”
This post, however, discusses only the accuracy of ultrasounds in pregnancy, so you will need to research potential risks and benefits as you make your decision on whether to have an ultrasound or not.
The most common use of ultrasounds is to estimate fetal weight. This article, published in the Journal of Ultrasound in Medicine discusses the accuracy of ultrasounds in measuring fetal weight. It discusses the “learning curve” that residents have, and notes that experienced (24 months+) residents have a 73.6% accuracy rate to within 10% of the actual weight. That sounds pretty good–within 10% is pretty small, right? Well, that actually works out to a difference of about a pound. My personal experience (an ultrasound performed 12 days before my baby was born), the estimated fetal weight at that time was 7 lb. 3 oz.; but his actual birthweight was 7 lb. 5 oz. Statistically, babies gain an average of half a pound a week at the end of pregnancy, which means that my baby gained nearly a pound, so had he been born the night of the ultrasound, he likely would have weighed 6 lb. 5 oz.
So what’s the big deal? Well, ultrasounds are most accurate when the weight is about average; if you get too far on either side of that–large or small–the accuracy rate goes down. Since doctors are more likely to induce your labor or perform a C-section if they suspect the baby is large, then accuracy counts a great deal. On the other end of the spectrum, if your baby seems to be small, your doctor may think s/he may be suffering and induce or section you, even if everything is just fine. One of my sisters-in-law had a small baby, so her next pregnancy was closely monitored by ultrasound (since having one baby with IUGR increases the likelihood of your next baby having it too). Her last ultrasound before the baby was born, the doctor couldn’t get a very good look at the baby because of her position in the uterus, but said that the baby would be lucky to be 6 pounds. The baby was actually my sister-in-law’s largest baby, weighing about 8 pounds. I’ve heard many other stories of women whose fetal weight guesstimates were similarly wrong–many of them ending up having a C-section “because the baby was so big,” only to find out afterwards that the baby was normal size.
What about the accuracy of other things ultrasound looks at? When I was first pregnant a few years ago, one of the (many) pregnancy and birth-related books I read was discussing ultrasound, and discussed a woman who had an ultrasound and was told her daughter had no kidneys. Since the baby would die soon after birth, the woman just decided to have an abortion, rather than carry the baby to term, all the while knowing that she could not live. Post-abortion, they examined the baby’s body and found that her kidneys were perfectly formed. They just didn’t show up on the ultrasound.
Here are a few stories from one of the email lists I am on, when we recently discussed this topic:
“She had an u/s and was told the baby had anencephaly (no brain) and would die within minutes of being born. She decided against an abortion/termination. She was going with her gut. They FORCED her to have psychological counseling due to her decision. The baby was born – perfect. This kid is smart as a whip and not one darn thing wrong with him.”
And another one:
“She was also a teenage mother…and was told that her ultrasound showed “birth defects”…she courageously told the technician that if her baby had no legs..she would be “legs” for the baby…or “arms” for the baby…or if her baby was brain damaged she would teach her baby as best she could and would love her baby no matter what. The ultrasound was WRONG! The baby with the “birth defects” is now 16yrs.old and taking Honors in most of her classes.. She is beautiful .. inside and out…“My friend Prof.Ian Donald, invented the Ultrasound as we know it today…but he must be turning in his grave at the way it is being “mishandled” and “misused” The accuracy is only as good as the person doing the scan… and the person reading the scan…and the machine itself. All of which are subject to human error.”
Some time ago on another list, a woman posted her own story. Since she was over 40 (when genetic problems are more likely with babies), she had an ultrasound a week or so before her baby was due, to see if the baby had Down Syndrome, or if it had a heart problem (many babies with genetic defects have heart problems). She had a Level-III ultrasound, and the doctor took 90 minutes on the scan, and finally ruled out either a heart problem or Down Syndrome. Within a few days, the baby died in utero….of the heart problem she was officially cleared from. When the baby was born a few days later, it was obvious she also had Down Syndrome.
No test is 100% accurate.