Pregnancy tests come in two varieties: urine and blood tests. Both of these check for the presence of a chemical called human chorionic gonadotropin (hCG) that is produced by the pregnancy and enters into the mother’s bloodstream and urine.
HCG is first detectable in the woman’s blood or urine approximately two weeks after conception. Since conception occurs at the midpoint of the four-week menstrual cycle, pregnancy tests first become positive at about the time a woman misses her period and first suspects that she might be pregnant.
A woman can take a pregnancy test to see if she is pregnant as soon as she misses her period. But it’s a little more complicated than that, for the following reasons:
Not all women’s bodies are exactly the same, so pregnancy tests in some women may not become positive until about a week after the missed period.
Blood tests are more accurate than urine tests, especially early in pregnancy. So, in the first few days after the missed period, a urine test might be negative when a blood test is already positive.
A positive pregnancy test (blood or urine) means that a woman is pregnant, but does not tell where the pregnancy is located (in the uterus, where it should be, or an ectopic pregnancy in the fallopian tube), or whether the pregnancy is developing normally. Ultrasound can provide this information one to two weeks after a missed period by showing where the pregnancy is developing and whether or not the embryo’s heart is beating.