Finding a Due Date with Ultrasound

• Many prenatal caregivers perform an ultrasound in early pregnancy to get an accurate due date. You might be wondering, "How does an ultrasound know when my baby is due?" In early pregnancy, just about every baby grows at the same rate. Learn how doctors and midwives use an ultrasound machine to date a pregnancy.
  
  

The Beginning

• In early pregnancy, you may be given an ultrasound at a regular prenatal appointment, or one may be scheduled for you. It's generally not necessary to do anything other than show up. Ultrasound technology has evolved, and it's no longer necessary to arrive with a full bladder, thankfully.
  
  

• The ultrasound technician will apply a water-based gel to your abdomen. He then uses the ultrasonic wand like a computer mouse. He clicks on each end of the baby's thigh bone, head and abdomen. The amount of amniotic fluid is also measured. In each stage of pregnancy, amniotic levels are predictable, and variances can indicate chromosomal abnormalities or organ anomalies.
  
  

• The measurements of the thighbone, abdomen (tummy) and head enter the computer system of the ultrasound machine and are automatically compared to statistical figures that are programmed into the system. The computer determines the gestational age (how old the fetus is) based upon comparisons. Later in pregnancy, heredity, medical conditions and nutrition can impact the way baby grows. Using ultrasound technology later in the pregnancy is a less reliable indicator of baby's due date.
  
  

• An ultrasound technician isn't allowed to diagnose medical conditions, and some doctors like their techs to be very tight-lipped. Others, however, will be allowed to point out your baby's head, heart and spine. Theoretically, the doctor must go over the images from the scan before giving you the results.
  
  

• Since there are several other factors that influence the way a baby grows, it's not possible to estimate the accuracy of prenatal ultrasound to date a pregnancy. It is considered to be an accurate way of dating a pregnancy when performed before 12 weeks gestation.