F&*king Pregnancy Tests, How Do They Work?

May 6, 2021 3:21 AM

I understand, or at least I think I understand, how pregnancy tests work in a general sense. What I don't get is why they're all labeled as being accurate within a certain number of days before or after you either "expect" or "miss" your period. I would like someone to explain to me why the tests' effectiveness depends on where you are in your menstrual cycle rather than how long it's been since you conceived.

This is partly idle curiosity and partly relevant to my life and anxieties. I have an irregular, often abnormally long (about 6 weeks on average) menstrual cycle and a (recent, traumatic) history of ectopic pregnancy. As a result, I spend a lot of time each month uncertain whether I have missed my period or am just still waiting for it. During this time, I sometimes take a pregnancy test out of an abundance of caution.*

Is the test pretty much guaranteed to give me a negative result every time because it's been less than my average length of cycle since I last menstruated? Or is it possible for it to give a positive result, if I were pregnant, four weeks after my last period? Or is there another answer? Why is the answer what it is?

Answers from people with OB/GYN or endocrinological background especially appreciated, but I'm open to as many explanations as I can get my grubby hands on.

Thank you in advance for helping me understand what's going on with the little pieces of paper I might be peeing on in vain!

*I am on the best birth control method available to me, and it has already failed me once, resulting in the aforementioned ectopic pregnancy. It also increases my risk of ectopic pregnancy in general, so I'm very concerned about catching any pregnancies as early as possible to reduce the risk of life-threatening complications.

posted by cabbage raccoon to Health & Fitness (10 answers total)

Pregnancy tests absolutely work the way you think they would - based on the time (ie amount of Hcg produced) since conception. If you hang around fertility forums, you’ll quickly get a feel for which tests are the most sensitive and will give the earliest results (last I looked, 10 days post conception was pretty reliable, but some might get a visible test result a few days sooner or later).

Edit - I didn’t mean you should hang around fertility forums; that’s just the source for my answer. If your cycle is regular, you could use ovulation tests for a cycle or two to get a feel for how many days in you ovulate, and then test 10-14 days after that day…
posted by Kriesa at 3:42 AM on May 6, 2021 [2 favorites]

It's as you think. The site Countdown To Pregnancy calculates the odds of a true positive based on days past ovulation if you know it, or approximates that based on cycle length if you don't. Those odds rise to very very nearly 100 as a hypothetical pregnancy advances and stay there the whole time. The site also has a lot of helpful information about them by brand. It does tend to assume that one wants to be pregnant and that all people capable of conceiving are women, but the data are still good.
posted by teremala at 3:58 AM on May 6, 2021 [1 favorite]

Best answer: Yes, that’s written for people who know when they’re expecting their period but don’t know when they ovulated.

Another way to dig into it is that a pregnancy test is almost completely reliable for sex that happened 15 or more days ago. Sperm can survive in there up to 6 days before ovulation, but mostly it’s sperm from 1-3 days before ovulation that fertilizes an egg. An embryo usually implants 8-10 days after ovulation, and most pregnancy tests are positive 1-2 days after that, depending on their sensitivity.

The suggestion to track your ovulation so you know when to expect your period could be helpful, but be aware that with irregular, longer cycles your body might gear up for ovulation and get a positive ovulation test more than once before actually ovulating some months. If you’re not using them already, the cheap bulk pregnancy and ovulation tests on Amazon are good.
posted by songs about trains at 4:36 AM on May 6, 2021 [5 favorites]

Best answer: The tests are based on time since conception. The possibly helpful thing for you is that ectopic pregnancy tends to have a rapid early increase in hCG, so would likely trigger a positive test sooner after conception.

Since your periods are irregular and you are concerned, you could consider basal body temperature monitoring. It can work pretty well to let you know when you have ovulated. Once you have a BBT thermometer and a way to track your temperature, it takes about a minute each morning.
posted by medusa at 4:36 AM on May 6, 2021 [2 favorites]

Another option other than using a BBT is to buy strips in bulk. And by bulk I mean packs of 50 or 100. They are really inexpensive and might help you to manage your totally legitimate worry.
posted by DarlingBri at 5:25 AM on May 6, 2021 [5 favorites]

Confirming that they work as you expect. I also have very irregular periods and first took a test four months into a pregnancy instead of the expected few weeks.

I'd like to caution that BBT and irregular periods don't work well together if you are using an app that starts over each time you note bleeding, especially if some of your bleeding is non-ovulatory. Apps usually start a new cycle when you note bleeding and increases in BBT can go unseen. This is why I didn't know to take a pregnancy test until I was four months along.
posted by peanut_mcgillicuty at 5:32 AM on May 6, 2021 [1 favorite]

FWIW, it's not just pregnancy tests, they use the same principle with dating pregnancy progress/due dates - it's really 38 weeks from conception to birth, but everyone uses 40 weeks from last menstrual period to birth, because most people have a better idea of their last period than their last ovulation. If someone is "6 weeks pregnant", they probably ovulated and had sex 4 weeks ago if their cycle is statistically typical. (If you are 1 week pregnant, you are .... not actually pregnant yet.) I don't know personally how due dates get adjusted for longer cycles - ultrasound measurements probably?
posted by february at 5:43 AM on May 6, 2021 [1 favorite]

Best answer: You might be better off ignoring your cycle completely, buying some bulk pregnancy tests and just taking one every 4 weeks or on eg the first of the month. If the test was positive you would be between about 2 and 6 (or so) weeks since conception. If not, relax. This suggestion assumes that you are not hoping to be pregnant and that the regular tests would provide enough reassurance that you don't worry about it between times (which may or may not be true).
posted by plonkee at 6:08 AM on May 6, 2021 [4 favorites]

Best answer: Yeah I was going to suggest ordering bulk tests and using them every three weeks or so (resetting when it doesn't make sense) as a way to relieve your mind of all this burden. Signed, someone who struggled with infertility for almost a decade who didn't have very regular periods.
posted by warriorqueen at 6:14 AM on May 6, 2021 [4 favorites]

Response by poster: I think I will take the suggestion of testing using bulk strips every 3 weeks so that I can be sure that if I do get pregnant I'll catch it 2-4 weeks after conception. I feel very well armed with understanding and a good plan now.

Thank you all for your kind, informative, and helpful comments! I
posted by cabbage raccoon at 5:58 AM on May 7, 2021