Fetal size and growth is important all through pregnancy, but it’s not always important for the same reason.
In the first trimester of pregnancy, fetal length is compared to your last period date to estimate how long the baby has been developing, and determine an estimated due date. As the pregnancy progresses, calculations of fetal size are used to assess how well a child is growing. Healthcare providers use these measurements to make sure that the fetus is developing as they should, and that there are no warning signs related to growth, as both too-small and too-large babies can be at risk for health problems.
Calculations of fetal size during later pregnancy are also compared against data about the average size for fetuses of that age. This way of looking at a baby’s age compared to their size will continue as they grow from fetus to infant, and all the way into toddlerhood, typically as the percentile they are in.
Percentiles can be useful for tracking to make sure a fetus’s, a baby’s, (and, later, child’s) growth is on-track and steady as they get older.
Early fetal measurements
In the first trimester, doctors measure fetal length from crown to rump, or from the top of a baby’s head to the bottom of their bottom. Crown to rump measurements are made using an ultrasound, and are only used early on in pregnancy (until about 14 weeks), while the fetus is still in more of a curled position.
Later fetal measurements
After 20 weeks of pregnancy, fetal growth is typically monitored using fundal height, or the distance from the pubic bone to the top of the uterus, essentially measuring the height of the uterus. After 20 weeks, fundal height in centimeters generally lines up roughly with gestational age, so that at 20 weeks, fundal height is generally about 20 cm. Fundal height is only a rough measurement of fetal size, but it’s a simple, noninvasive way of getting a sense of fetal size and growth.
Most healthcare providers use fundal height as a regular way of getting a rough sense of fetal growth, although some don’t find it as useful, since it can be less accurate for pregnancies of people who are obese, people who have a history of fibroids, or who are carrying twins or multiples. It can also be less accurate in pregnancies where there’s extra fluid in the uterus (called polyhydramnios pregnancies), or where there’s an especially large baby (called a macrosomia).
A more accurate, but also more involved, way of measuring fetal size after 14 weeks is by taking measurements using an ultrasound. After 14 weeks, their growth is estimated using measurements of other, newly-developed body parts. This type of measurement gives a more accurate sense of size. Using an ultrasound to measure the diameter, or distance across, and circumference, or distance around, the head, the circumference of the abdomen, and the length of the femur (thigh bone) gives doctors the ability to estimate fetal size more accurately in the later part of pregnancy.
Generally, routine ultrasounds to measure fetal size in low-risk pregnancies aren’t considered clinically useful, and are only used when a healthcare provider is concerned about the baby’s growth.
- Romy Gaillard, et al. “Tracking of fetal growth characteristics during different trimesters and the risk of adverse birth outcomes.” International Journal of Epidemiology. 43(4): 1140-1153. Retrieved June 20 2017. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4258770/. August 2014.
- Yvonne Butler Tobah. “What’s the significance of a fundal height measurement?” Mayo Clinic Mayo Clinic, March 4 2017. Retrieved June 20 2017. http://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/expert-answers/fundal-height/faq-20057962.
- “12 Fetal growth and wellbeing.” National Center for Biotechnological Information, U.S. National Library of Medicine. National Collaborating Centre for Women’s and Children’s Health, 2008. Retrieved June 20 2017. https://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0009600/.
- “Intrauterine growth restriction.” MedlinePlus. U.S. National Library of Medicine, October 4 2016. Retrieved June 20 2017. https://medlineplus.gov/ency/article/001500.htm.