Home Pregnancy Tests and Timing

Factual Questions
1

From all the home pregnancy tests I’ve seen recently, they all state that the accuracy of the test goes up the closer it is to the expected period. So, if you are pregnant, the test is most likely to be accurate on and after the date of the next (missed) period. Most tests claim over 99% accuracy when used on or after this date, with significantly decreasing accuracy when used before this date.

If you are pregnant, why does this date really matter so much in terms of these tests? I mean, if you get fertilized right after your last period ends you can theoretically be around three weeks pregnant when your next period was scheduled to roll around. On the flip side, you can also become pregnant right before this date and only be a few days pregnant instead. It doesn’t seem right to me that the tests would be 99%+ accurate for both of these cases, and I would actually assume that the latter case should have a much greater chance of false negatives than the former.

So I don’t understand what’s so special about the date of the expected period. Is it just used as a reference point for when it makes the most sense for a woman to test? Obviously, if you get your period as expected there would be no need to test after this date, which could conceivably raise the accuracy of the tests a significant amount as the only people testing are those most likely to be pregnant.

Does anybody have the straight dope on this aspect of these tests? Any general information on home pregnancy tests would also be appreciated, as I’m sure I’m not 100% clear on how these tests work.

My apologies if this has been asked before, the search function is down again. Also, before anyone gets confused, I’m actually a guy who was relieved with a negative test result yesterday and is curious about how these work.

2

It’s just a reference point where the levels of hcg in the blood can be expected to be detectable. It’s going to vary quite a bit from woman to woman. The hcg levels should double daily after fertilization, so there is an exponential rise in the levels. Fertilization could take place in a pretty wide window, but the expected period date should allow enough accumulation to detect it in the urine.

That being said, there is a huge false negative (not much false positive) rate. You’ll notice that they don’t explain what that 99% means, and I suspect it isn’t that it is accurate for 99 out of 100 women even in a laboratory setting.

Also, you wouldn’t get ferilized right after your last period, but about two weeks (plus or minus) after your last period. So you would, at most, be pregnant about two weeks at the time of the missed period. But, that’s assuming instantaneous implantation, which will usually take a few days.

3

I don’t know a ton about pregnancy physiology, but I’ll get the thread started:

Home pregnancy tests test for a certain level of the hormone hCG (human chorionic gonadotropin), which levels do not start to rise until after implantation, which typically occurs 3-4 weeks after the last menstrual period (1-2 weeks after ovulation). If you test while the little zygote/blastocyst is still floating around, you will get a false negative. However, as you note, time of fertilization can vary, as can time of implantation. Per womenshealth.gov, this claim of 99% accuracy on the date of the missed period is misleading, as 10% or more of women may have a false negative; accuracy is better a week after the start of missed period.
http://www.womenshealth.gov/faq/pregnancy-tests.cfm

Interestingly, IMO, it takes a while for progesterone levels to rise to a point where you can say, yep, that’s a pregnancy.

4

Once the egg implants the women start producing hCG. This is the hormone that pregnancy tests test for in the urine. The tests will only show a positive if it is past a particular threshold.

There is hCG in the urine starting around 7 days after implantation. It doubles approx every 48 hours.

So, if your pregnant, and you test too soon, you’ll get a negative. Wait 48hrs and test again to get the best shot of a positive.

5

AIUI, the exact day you get pregnant is not related to the day you did the deed, but to where you are within your cycle. If your cycle is regular, you’ll ovulate on roughly the same day every cycle, and be fertile for a limited period of time after that. Let’s pretend and say you ovulate 2 weeks before your period, and are fertile for 1 week after that. The pregnancy test assumes you are going to get pregnant in that 1 week period, give themselves a few days to a week for the hCG levels to change, then suggest that time as the best time to take the test.

To say it a slightly different way, you can’t get fertilized right after your period ends, because you haven’t ovulated yet. That’s not to say that you can’t get pregnant if you have sex then, but you won’t be pregnant that day, you’ll get pregnant at a later date, due to the sex on that day.

6

No, you generally can’t. There is this thing called a “luteal phase”, which is the span of time post-ovulation but pre-period. In healthy women, it’s about 12-14 days long. (There’s an infertility problem called a “short luteal phase” or “luteal phase defect”, but I won’t go into those right now.)

The luteal phase is remarkably consistent; women may vary a lot with their cycles, but the luteal phase is almost always the same, down to the hour. If you’re period is late due to stress or illness, it’s because of stress or illness in the first half, delaying ovulation. Once you ovulate, that’s it, it’s a ticking time bomb of progesterone and a dying ovum.

What this means, practically speaking, is that if you conceive, it’s because an egg has been released, which starts the clock ticking. The egg is only good for about 24 hours, but you’ve still got that luteal phase to get through before you menstruate. So you’ll never (in a healthy woman) menstruate less than 12 days after you ovulated. 12 days is more than enough time to build up enough HcG to show up on a test.

Actually, it works the other way 'round. You’re fertile for a little less than a week *before *you ovulate, and almost never after. This is because sperm can live, in good quality cervical fluid, for about 4-5 days. They use this time to swim up the uterus and into the fallopian tubes. By the time they arrive, a fresh egg is just being released into the fallopian tubes, where it can be fertilized. So on Monday you can ovulate and get pregnant from sex you had on Saturday night. Once fertilized, the egg/sperm (now called a blastocyst) rapidly divides as it moves back down the fallopian tube to the uterus. By the time it reaches the uterus, it’s formed two layers, one of which will become the placenta when it bumps into the uterus wall and implants there.

If you don’t have sex until you ovulate, or after, the egg is either going to be dead (again, it only has a fertile lifespan of about 24 hours) or it’s going to make it into the vast cavern of the uterus before the sperm find it. Even if the sperm manage to find it and even if it’s still viable, it doesn’t have time (as a blastocyst) to form those two layers, implant and make a placenta before menstruation starts and it gets swept out through the cervix with the menstrual fluid.

7

Actually, I was trying to describe the viability of the egg not the “fertility” of the woman, but my post was still inaccurate. Restating my post with your better information, a woman will ovulate roughly 2 weeks before her period, and the egg will be fertilized within 24 hours of ovulation. A short window of time where a woman can “become pregnant”, entirely based on the dates of her cycle, which is why pregnancy tests (and pregnancy dates) are cycle based instead of sex based.

8

Yes! That’s it exactly.

9

I think this more or less answered my question, particularly the part I bolded.

Thanks to everybody for the replies!