Pregnancy Testing

September 23, 2011

Early and accurate diagnosis of pregnancy is important in order to determine the estimated date of delivery and to allow the pregnant woman to make decisions regarding her reproductive care. There are several different ways to diagnose intrauterine pregnancy including increased blood or urine levels of the pregnancy hormone betahuman chorionic gonadotropin (beta-hCG), ultrasound visualization of an embryo in the uterus, and the presence of audible fetal heart tones.

Within 12 days after conception, the placenta, a collection of fetal and maternal cells, begins to form and secrete the hormone beta-hCG. The levels of betahCG double every 2-3 days and are detectable in both blood and urine. Concentrations of this hormone vary depending on the individual and the stage of pregnancy.

Home pregnancy tests use monoclonal antibodies to detect the presence of beta-hCG in the urine. Some tests are able to detect very low levels of this hormone (approximately ten millionths of an International Unit) several days before a missed menstrual period. However, due to the very low levels of hormone in the urine and inherent inaccuracies of the test, up to 50% of early pregnancies may be missed if a woman depends only on this early urine testing. Home urine pregnancy tests are most accurate if taken 1 week after the missed menstrual period. A positive home pregnancy test should be followed up by an appointment with a maternity care provider (family physician, midwife, or obstetrician) to confirm the diagnosis of pregnancy and discuss ongoing care.

While urine tests are designed to detect either the presence or absence of beta-hCG, serum pregnancy tests can be used to quantify the amount of this hormone in the blood. The absolute amount of beta-hCG may be used by a woman’s health care provider to follow the progress of the early pregnancy. A level that does not double every 2-3 days may be suggestive of a nonviable pregnancy.

Ultrasound of the uterus can provide confirmation of an intrauterine pregnancy as well as an accurate estimate of the due date. Typically, by 6 weeks’ gestation (4 weeks after conception), a transvaginal ultrasound can detect an embryonic pole and heartbeat. It is possible to hear the fetal heartbeat through the abdominal wall using a Doppler device at 10-12 weeks’ gestational age (8-10 weeks after conception).

SEE ALSO: Pregnancy, Prenatal care, Ultrasound

Category: P