Since most women don't know the exact date of conception, the standard method for charting a pregnancy is by counting forward from the first day of your last menstrual period (LMP). This method assumes that you ovulate 14 days after your period begins (and that you'll remember the day your period started!). These results give health care providers -- and you -- a ballpark estimate for your baby's due date.Your First Ultrasound
American Institute of Ultrasound in Medicine - AIUM.org
For more accurate pregnancy dating, health care providers can request an ultrasound. Careful measurements taken in the first trimester are even more accurate at predicting an embryo's age than ultrasounds performed later in pregnancy. Using these measurements, a sonographer can pinpoint a baby's age to within a few days. As the baby-to-be grows larger, these measurements are less reliable at predicting a baby's age because of varying growth rates. Just like kids come in different shapes and sizes, in utero babies can grow at different rates, too.
Along with figuring out your due date, your health care provider might ask for an early ultrasound for a variety of reasons: to confirm your pregnancy, to chart the baby-to-be's heartbeat, or to troubleshoot if you've been experiencing pelvic pain or vaginal bleeding.
You might already be familiar with ultrasounds from popular TV shows or from attending exams with expectant friends. These ?transabdominal? ultrasounds are performed by placing cool gel across the belly and using a transducer, which emits sound waves, to generate a picture of your developing baby. Later in pregnancy, this method works well. But early in pregnancy, when the embryo is still tiny, the sonographer will most likely need to do a ?transvaginal? exam.
With a transvaginal ultrasound, the sonographer places the transducer into the vagina. The transducer will not only be able to grab pictures of your growing baby, but it will also produce images with greater detail than with a transabdominal exam.
Early, accurate dating of a baby's age and expected delivery date might help your health care provider make decisions about how to manage your pregnancy. For example, for women who go into labor early -- or late -- your provider will have an accurate picture of your baby's age. And this early peek might reveal possible genetic abnormalities. Accurate dating is also important for the timing and interpretation of some specialized prenatal tests, such as the quadruple screen, nuchal translucency measurements, and others used to assess the likelihood of genetic abnormalities.