Purpose of Ultrasound
Ultrasound pictures are created by bouncing high frequency sound waves off body tissue. A transducer is used to convert energy into sound waves and then record pictures on the ultrasound screen. Ultrasound is often used in pregnancy, not only to check for fetal abnormalities but also to establish an accurate due date. A study reported in Ultrasound Obstetric Gynecology concluded that 92 percent of pregnancies deliver between 37 and 42 weeks when dated by ultrasound as compared to 87 percent when dated by last menstrual period.
Early Dating
The earlier an ultrasound is done in pregnancy, the more accurate the dates. According to the Family Practice Notebook, a gestational sac is first seen at four to five weeks of pregnancy. Gestational age can be figured by adding height, width and length of the sac and dividing by three. A yolk sac is first seen at five to six weeks and is present until around 10 weeks.
Crown-rump (C-R) length is the most accurate method of dating a pregnancy. Crown-rump measurements are accurate between seven and 13 weeks, and measure 0.4 cm at six weeks, increasing to 8.0 cm at 14 weeks. Fetal heartbeat is seen between six and seven weeks, when the crown rump length measures 5 mm.
First trimester ultrasounds may be better viewed if the ultrasound is done transvaginally rather than abdominally, especially in cases of maternal obesity or uterine fibroids.
Second Trimester
Dating is most accurate in the first 12 weeks, but if a patient doesn't have care until later in pregnancy, measurements can be taken that give a fairly accurate idea of pregnancy duration. Three measurements can be taken, and the most accurate method of dating is to compare the fetal head circumference and femur length. Abdominal circumference can also be checked.
Bi-parietal measurement of the fetal head can be done around 13 weeks, and should be done as early as possible, as babies have different sized heads at the same weight in late pregnancy. Bi-parietal diameter is measured as the difference between two sides of the head, and should measure 2.4 cm at 13 weeks.
The femur, the longest bone in the body, should measure 1.5 cm at 14 weeks. Abdominal circumference shouldn't be used to date a pregnancy; it is a valuable measurement for late pregnancy, to measure fetal size and weight.
Third Trimester
Dating a pregnancy in the third trimester is less accurate than early dating, but an article published in "Ultrasound in Obstetrics and Gynecology" suggests that taking bi-parietal and femur length measurements is more accurate than once believed. Late screening may be the only tool available in determining due dates in women who haven't had prenatal care. Dating pregnancy in the second and third trimesters is easier in thin women and women with adequate amniotic fluid.
