Accuracy of Pregnancy Tests

Posted on Sep 18

There are two main types of pregnancy tests given: One uses a urine sample, and the other uses a blood sample. Both tests detect the presence of a hormone called Human Chorionic Gonadopotrin (hCG). After the fertilized egg has attached itself to the uterine lining (implantation) the placenta starts producing this hormone. It builds up rapidly in the first few days, and is the cause of most pregnancy symptoms. Typically, the hCG levels will double every 72 hours. The level will reach its peak in the first 8-11 weeks of pregnancy and then will decline and level off for the remainder of the pregnancy.

Implantation of a fertilized egg normally occurs between 6-12 days after ovulation. Ovulation is different for each woman, but is usually between 11-21 days from the first day of the last menstrual period (LMP). There are two phases of ovulation: the follicular, which begins on the first day of the LMP until ovulation, and the luteal, which is from the day of ovulation until the next menstrual period begins, usually 12-16 days. (Outside factors like stress and illness can throw off your ovulation, and ultimately change the time of your next menstrual period.)

URINE PREGNANCY TEST:

Urine pregnancy tests are the most common tests used. Home pregnancy tests are generally about 97% accurate. The Urine hCG pregnancy tests used by Pregnancy Care Center of Chandler and Gilbert are >99% accurate, capable of detecting pregnancy as early as 10 days after ovulation, although waiting until a menstrual period is missed is recommended. A positive test result most likely indicates pregnancy. False positives are extremely rare. However, there are some conditions that may cause a false positive, such as certain types of cancer and early miscarriage. Some antibiotics may also interfere with test results. A confirmed pregnancy should only be made by a physician after all clinical and laboratory findings have been evaluated.

BLOOD PREGNANCY TEST:

There are two types of blood tests. A quantitative test measures the exact amount of hCG in the blood and a qualitative test gives a simple yes or no answer as to whether there is a pregnancy or not. A blood pregnancy test can detect a pregnancy a few days earlier than a urine pregnancy test and can measure the concentration of the hCG hormone, but also requires going to a doctor or lab, and the results take longer. Blood tests may be done when a woman’s urine pregnancy tests show negative but she has continued pregnancy symptoms.