How Do Doctors Know the Size of a Baby During Delivery?

By Candice Coleman
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From your first prenatal appointment, your physician will make estimates about your baby's size and growth. Based on these results, your doctor can get a picture of your baby's health and intervene if your baby's size might cause complications during delivery. Several techniques are used to estimate your baby's height and weight.

Weight Gain

Most full-term babies come into the world weighing about 7 pounds, according to Boston Children's Hospital, but birth weights vary widely, and a mother's weight gain during pregnancy can influence it. Doctors might believe a baby will be larger than average at delivery if a mother gains weight too rapidly during her pregnancy or gains too much weight overall, according to the March of Dimes. Limited weight gain can have a similar effect -- doctors might predict a smaller-than-average baby.


Most women will get at least one ultrasound during the course of their pregnancies, according to KidsHealth. This test might not only reveal the baby's sex, but also give the doctor an idea of the baby's growth. Technicians coat your stomach with gel before running an instrument over it to get a picture of your unborn little one. Though most ultrasounds are performed between 18 and 20 weeks, some women might receive multiple ultrasounds during the course of their pregnancies.


Another way doctors estimate the size of your baby at delivery is through the use of fundal height measurements, according to the Mayo Clinic. The measurement is usually taken at each of your prenatal appointments. A doctor will measure from the top of your uterus to your pelvic bone and compare it to the average measurement. Major differences between your measurement and the average might lead your doctor to expect a significantly smaller or larger-than-average baby at delivery.

Additional Information

Predicting a baby's size and weight at delivery is not an exact science, and doctors might find that their estimates were off when your baby arrives. Several factors, including multiple pregnancy, might make it difficult to accurately estimate a baby's size, according to the Mayo Clinic. If you or your doctor have concerns about your baby's growth, ask what you might be able to do to get your baby's development back on track.

About the Author

Candice Coleman worked in the public school system as a middle school and high school substitute teacher. In addition to teaching, she is also a tutor for high school and college students.