Whether you are hoping to see two lines on the stick that mean you are pregnant, or if your goal is to see a negative test and know that you are definitely not pregnant, knowing exactly when to take a pregnancy test can be a big deal. For most women, the earlier they can know the better, but there is such a thing as starting too early.

Home pregnancy tests work by screening your urine for a hormone called human chorionic gonadotropin, or hCG. This is made and secreted by an egg that has been fertilized and has then implanted in the wall of the uterus. While a woman’s menstrual cycle can vary, the following days can provide a helpful guide:

  • Day 1 is the first day of your period
  • Day 14 is usually when ovulation occurs (when an egg is released)
  • If fertilization is to happen, it is usually shortly after ovulation
  • Day 21 is about the time that a fertilized egg will implant in the wall of the uterus (for most women who have 28 day cycles, this is 1 week before they expect their next period)

Therefore, day 21—or 1 week before you expect your next period—is the approximate time when hCG would be secreted and could be detected by a urine pregnancy test. However, just because you peed on a stick and it was negative on day 21 doesn’t mean you aren’t pregnant.

Your test could be negative if you are pregnant at this point if the hCG levels are too low to be detected by the test. Yes, a pregnancy test can be wrong. Different over-the-counter tests have varying levels of hCG they screen for, so you can read the instructions to figure out which one detects the lowest levels. In general, these tests will come back positive if your hCG levels are at least 25-50 mIU.

If you tested a week before your period and got a negative result but want to be completely sure, there are a few things you can do. You can re-test at home in a few days or wait to see if you get your period. If the idea of waiting is too hard for you, you can call your doctor or midwife and ask that a blood pregnancy test be done. This will detect hCG at any level, so it is positive much sooner.

Keep in mind that having a negative result early on followed by a positive test does not mean your hCG levels are too low or that something is wrong with this pregnancy. It could be that you ovulated on a different day than you thought or that implantation happened at a different time.

Written by Jennifer Lincoln, MD, IBCLC, Board Certified OB/GYN

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