You may find out at a routine prenatal visit that your practitioner thinks your uterus is measuring either too big or too small. This finding isn’t cause for immediate alarm. Often in this situation, your practitioner suggests that you have an ultrasound exam to get a better idea of how big the baby is.
Ultrasound is used to measure parts of the baby — the head’s size, the abdomen’s circumference, and the thighbone’s length. Your practitioner then plugs these measurements into a mathematical equation that gives the estimated fetal weight (EFW).
That estimate is then entered on a curve plotting the baby’s age in weeks against weight, which represents the average growth of thousands of fetuses at each gestational age.
Your practitioner can check to see where your baby’s weight falls on the curve and thus tell which percentile the baby is in. If the baby’s weight is anywhere between the 10th and the 90th percentiles, the weight is considered normal. Remember, not every baby is at the 50th percentile, so the 20th percentile is still normal and no reason to worry.
Keep in mind that although ultrasound is an excellent tool for assessing fetal growth, it isn’t perfect. Judging the baby’s weight by an ultrasound exam isn’t the same as putting the baby on a scale. Weight estimates can vary by as much as 10 to 20 percent in the third trimester due to variations in body composition. So if your baby is outside the normal range, don’t worry.
If your baby measures very large (macrosomia), your practitioner may suggest you have another glucose screen to check for gestational diabetes. If your baby measures especially small (intrauterine growth restriction), your doctor may suggest you be followed more closely — that you undergo non-stress tests and repeat ultrasound exams to keep an eye on fetal growth.
