Measuring the size of a baby plays a critical role in prenatal care. Doctors use estimates of a baby's size to determine fetal age, identify problems such as intrauterine growth restriction -- where a baby grows too slowly -- and induce delivery if they suspect a baby is large for gestational age. While no method of measuring a baby's size in the womb is perfectly accurate, doctors can use several methods for estimating fetal size.
Determine your baby's length from crown to rump between 7 and 14 weeks with an ultrasound scan. During the first trimester, ultrasound allows calculation of a baby's size by measuring the length from the top of her head to her bottom, since babies at this stage tuck their legs into their chests. This method provides the most accurate estimate of a baby's age, with an accuracy rate of plus or minus three days at seven weeks.
Measure your fundal height -- the distance between the top of your pubic bone and the top of your uterus. From 20 weeks on, your doctor will feel your uterus at each appointment to see how much it has grown. The fundal height should be approximately the same number of centimetres as your weeks of pregnancy. Fundal height doesn't provide a very accurate measurement, since it can be thrown off by the amount of amniotic fluid, the presence of multiple foetuses and a baby's position. Breech babies tend to measure too large, and babies that have dropped or are turned sideways measure too small. However, since the test is free, it provides a simple method for roughly estimating whether a baby is growing at the correct rate.
Calculate your baby's size using an ultrasound measurement of femur length, abdominal circumference and biparietal diameter -- the distance between two skull bones -- between 14 and 26 weeks. Some doctors may also take other fetal measurements, such as head circumference, humerus length and occipitofrontal diameter -- another measurement between two bones in the fetal head. If one or more measurements differ from the others, an explanation should be found for the difference rather than averaging the measurements. For instance, a smaller biparietal diameter measurement may simply indicate a baby has a flat head.
Starting in the second trimester, babies begin growing at different rates, so a baby's size is a less accurate indication of age than in the first trimester.
No method for accurately estimating fetal weight exists. Using biparietal diameter and abdominal circumference to determine weight leads to an error rate of 10 per cent. While many doctors use ultrasound to estimate a baby's weight near the end of pregnancy, research does not support this practice because the accuracy rate is so low.