In a low-risk pregnancy, the abdomen is measured at prenatal visits to assess the baby's growth. The measurement in centimeters from the top of your pubic bone to the top of your uterus (the fundus) should be about the same as the number of weeks you are pregnant, with an allowance of up to 2 cm either way. For example, if you are 26 weeks' pregnant, you should measure between 24 and 28 cm. Your fundal height can be measured between 24 and 36-37 weeks, since once your baby "drops" into the pelvis in late pregnancy, the measurement may not reflect his or her true size. If there is a variation of 3 cm or more, your doctor will arrange for an ultrasound to check your baby's growth (see Growth charts) and the amount of amniotic fluid. If the scan indicates a problem, the doctor will arrange for scans every two weeks since analyzing growth patterns over time gives a more accurate assessment of whether your baby's growth is normal.
In certain cases, for example if a woman is obese, in twin pregnancies, and where there are large uterine fibroids, the only accurate way to measure growth is by ultrasound.
By late pregnancy, the length of your baby can no longer fit on the screen and so his or her size will be calculated by combining several measurements in a mathematical formula, as is done at the 20-week scan (see Your 20-week Scan). Measurements include the width of the head (biparietal diameter), circumference of the head, and the abdominal circumference, which are plotted on a graph over a period of time (see Growth charts). The length of the upper leg bone (femur length) may also be measured. If your baby is smaller than the 10th percentile or has a small abdomen, you may have more tests to assess his or her well-being (see Your baby's well-being). A measurement above the 90th percentile can be a sign of gestational diabetes and will need investigating. Also, if your baby is at the larger end of the scale, your doctor may recommend you have a cesarean delivery.