How a Pregnancy Test Works and Which Test to Buy

Before starting on the basic physiology of the female hormones during egg implantation I want to say a quick word the pregnancy products available. In general, any home diagnostic test is regulated as a medical device by the FDA (center for devices and radiological health). In order to be marketed to the general public, home test kits must be at minimum 95% accurate, permit proper and simple use to everyone regardless of intelligence, list all interfering substances, food, and medications, and include easy to use instructions.

NOTE: only use tests that have been cleared by the FDA

At home pregnancy test allow woman to make early rational decisions about their pregnancies, including proper prenatal and nutritional care as well as appropriate lifestyle choices. The symptoms associated with pregnancy include late/missed periods, nausea, vomiting, fatigue, and breast tenderness. It is usually the missed period that provoke women to seek the purchase of a pregnancy test.

A pregnancy test detects human chorionic gonadotropin (hCG) in the urine. Egg implantation usually begins about 5-6 days after fertilization (the sperm fuses with the egg and forms an embryo). Once the embryo has been formed it begins to release hCG. After about 5 days hCG can be detected in the blood, but it takes about 1-2 weeks after fertilization for the hCG to be detected in the urine.

Each test has varying sensitivities (true positivity). The highest possible sensitivity on the 1^st day of missed period is approximately 90%. The highest possible sensitivity one week after the 1^st day of missed period is approximately 97%. Overall, the earlier you test, the less likely the test will be accurate.

False Positives (says you are pregnant but you really are not)

• urine collected in a wax paper cup/soap residue
• recent pregnancy, miscarriage, or abortion (in past 8 weeks)
• certain fertility medications

False Negatives (says you are not pregnant but you really are) - most common

• dilute urine (always use first morning urine as it is the most concentrated)
• testing too early (not enough hCG)

Finally, if the test reads negative and there is still no pregnancy, repeat the test in one week. If second test is still negative after 1 week and still no period, a trip to the physician is warranted.

Random notes on pregnancy:

1.) Oral contraceptives do not interfere with pregnancy tests

2.) Taking a multivitamin at the time of conception has been shown to decrease the deleterious effects of morning sickness in some woman.

3.) Folic acid and vitamin B12 must be taken during the course of the pregnancy and at the time of conception to prevent neuronal tube defects in the baby

Special thanks to Dr. Kimberly Schumde PharmD professor at the UT college of Pharmacy