A: Dear Brenda,

Most babies do weigh more than 2.5 pounds at 30 weeks. The average fetal weight at 30 weeks is around 3.5 pounds. 90% of babies weigh between 2.6 pounds and 4.8 pounds. This means that out of 100 babies measured at 30 weeks, only 5 would weigh less than 2.6 pounds.

Sometimes when an ultrasound shows a baby to weigh less than average, it may be that the pregnancy is not as far along as initially thought. So, the first question I would ask is—are you and your doctor sure about your dates? Did you record your last menstrual period and use this to calculate your due date? Did you have prenatal care early in the pregnancy, and did your doctor think your uterine size matched your menstrual dates? Did you have any prior ultrasound to document the fetal size and age?

Second, I'd want to know more details about the recent ultrasound. The fetal weight is estimated by a calculation using measurements of your baby's head, abdomen and leg. Were all of these measurements concordant – that is, do all of the measurements appear to be smaller than average? Or, are there discrepancies in the measurements. For example, some babies have an average head size and length, but are thinner than average and will have a small estimated weight. These babies may not be getting adequate nutrition during the pregnancy. This may occur in pregnancies with maternal diabetes or high blood pressure.

Third, I'd want to know about your baby's pattern of growth in this pregnancy. Comparing your recent ultrasound to a prior ultrasound would be very helpful. If your baby was previously documented to be of average size, and now appears to be smaller than average, it could suggest that the baby is not growing optimally.

There are additional tests that your doctor can use to assess your baby's well-being. These include fetal heart rate monitoring, studies to check the flow of blood in the umbilical cord, and further ultrasounds to measure amniotic fluid and fetal growth. If these tests suggest a baby is not receiving adequate blood flow from the placenta, inducing labor may be advised.

Obviously, it is very important for you, your partner and your doctor to talk in depth about the significance of your recent ultrasound results. Together, you can discuss what steps to take next. Your doctor should let you know how soon further testing should be done and whether a consultation with a perinatologist would be helpful.