Fundal Height

• Though high-tech options allow fetuses to be measured very accurately, some doctors opt not to use such technology on low-risk mothers. Instead, doctors measure the fundal height of the mother and use it as a proxy for fetal size. To measure the fundal height, a doctor will take a tape measure and measure from the top of the woman's uterus to her pubic bone. Once a woman is into her second and third trimesters, the fundal height in centimeters should be roughly equal to her weeks of gestation. So, a woman who is 28 weeks along should have a fundal height of 28 centimeters. In such cases, doctors predict that the fetus is growing properly and may not require the woman to undergo further testing. However, fundal height provides only an estimate of fetal size, and better technology now exists to give a more accurate assessment of the health of a fetus.
  
  

Ultrasound Technology

• While some expectant mothers may not have any ultrasounds during their pregnancies, today most doctors opt to do at least one anatomy ultrasound around 4 ½ months gestation. Pregnant women may receive a transvaginal ultrasound early in their pregnancy. These ultrasounds locate the embryo and ensure that it is properly located in the woman's uterus. More commonly, women get topical ultrasounds halfway through their pregnancies. These ultrasounds use soundwaves to derive visual images of the fetus. Ultrasound technicians can use these computer-generated images to take measurements of the fetus.
  
  

Crown to Rump

• During an ultrasound, the technician measures the fetus in a variety of ways. The most common and meaningful measurement early in a pregnancy is the crown-to-rump length. This length can estimate gestational age and can determine if the fetus is growing at the proper rate. Typically, this measurement is an accurate assessment---that is, within 5 days---of true gestational age.
  
  

Other Important Measurements

• The biparietal diameter is another important measure of fetus development. It measures the distance between the two sides of the head. It is more accurate early in pregnancies since fetuses progress at different rates in the last trimester. Doctors also measure femur length and abdominal circumference via ultrasound to ensure that the baby is developing properly. Some problems discovered during an ultrasound can be treated prior to birth.
  
  

Less Common Measurements

• Women who are considered high-risk will likely undergo an additional battery of tests throughout their pregnancies. They are often tested for uncommon diseases and problems in the fetus. Doctors test these high-risk fetuses using less common measurements that serve as indicators of rare diseases. For example, ultrasound technicians measure the yolk sac diameter early in a pregnancy. An absence of a yolk sac indicates fetal demise and an abnormally large yolk sac can indicate potentially severe problems. Other less common measures are gestational sac diameter and head circumference.