Why do some women get early positive pregnancy tests, while others have to wait?

This is a question I've seen a lot on TTC forums and also wondered myself. Now I understand the process behind it, I thought it might be useful to explain it to others.

So, why do some women get positive pregnancy tests as early as 10 or even 9 dpo (days post ovulation), around 5 days before a missed period, while others can have their expected period date sail by with no sign on any tests, putting them horribly into the dreaded limbo - not confirmed pregnant, not confirmed non-pregnant, what is this, Schrodinger's uterus?

As it turns out, there are several factors at play.

1. Base hCG level.

A hCG level of 5 or higher is considered a sign of pregnancy. But most women produce a small amount of hCG all the time. Just somewhere under 5. Because hCG increases by doubling, your base (non-pregnant) hCG level is fairly instrumental in how fast your hCG will reach the level that it's detectable by a test.

2. Sensitivity of the test.

There are two commonly available types of pregnancy test. Most standard tests detect hCG over 25. Some more sensitive tests will detect a level over 10. By the way, not all tests sold as "sensitive" are the more sensitive kind. Check the packaging. You're looking for "10 mIU" or "25 mIU". And these are a rough guideline - there's a margin of error.

3. Implantation timing and luteal phase length.

The body only begins to increase hCG production in response to pregnancy from the date of implantation, not the moment where sperm meets egg. Implantation normally happens around 7-10 days after ovulation. So if you're counting from ovulation, this counts. Additionally, if you have a short luteal phase, you're likely to have implantation happening towards the end of it, meaning you'll have to wait longer after a missed period to see a positive test. If you have a typical or longer luteal phase, then implantation will happen longer before the missed period and hCG might have built up enough during that point for a test to be positive on or before the missed period day. You don't need to know your luteal phase duration, but it can provide some extra information if you do know it.

4. hCG doubling time.

When you're pregnant, your body will keep doubling the amount of hCG it produces. The normal rate of doubling is anywhere from 36 to 72 hours - ie, it will take anywhere between 1.5 and 3 days for the hCG level in your body to double. Figures outside of this rate point to problems, but anything within this range is normal and healthy. In those early days of pregnancy, the numbers are so small and the days seem so long that every day counts.

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So... bring in my highly unscientific graph. This is an estimate based on figures I've tried to get as accurate as possible. It shows the hCG increase of two imaginary women, Ms. A (in pink/mauve) and Ms. B (in violet/dark blue). They both know when they ovulated and are expecting their period 14 days later, which is average.

Ms. A is lucky. She has a relatively high natural non-pregnant hCG level (4), her pregnancy implants on day 6 post ovulation, and her body starts multiplying hCG at the fastest rate. By just 8dpo, her hCG levels have reached about 11 or 12 meaning that they can already be detected on some of the most sensitive tests (shown by the orange line). This is where the claims come in that some tests can detect pregnancy from 6 days before a missed period. It's true, but not all pregnancies. If she's using a standard, less sensitive test, (yellow line) she'll have to wait a little longer, but she's still likely to get a positive result by 10dpo, four days before her missed period. And she'll keep getting positives from this day on.

Ms. B, though, is stuck in limbo. She has a lower pre-pregnancy hCG level of 1, her pregnancy doesn't implant until day 10 post ovulation, and her hCG is slower to multiply. Her pregnancy is still perfectly healthy, but she'll see her expected period date come and go and keep getting negatives until possibly five days later when she might get a faint line on a sensitive test, she'll have to wait another day or two for a clear one, and a full ten days after her expected period date until she gets a positive on a standard test.

Of course, these are two extremes of the scale. Most women will fall somewhere in between these examples and additionally not everyone gets their period exactly 14 days after they ovulate. Plus, just to make things even more complicated, the tests aren't exactly accurate to the 10 or 25 mIU hCG level either... the point is, there is a huge variance on when different women get a positive pregnancy test result. Don't panic if you don't have one. You are not out of the race until you get your period.

Bonus question: Does the darkness of the line matter? Should my tests be getting steadily darker?

It does... and it doesn't. If you're somewhere on the cusp of the level of hCG the test can detect then you'll get a faint result, sometimes barely even visible (often referred to as a "squinter"). This will tend to get darker as you get closer to the amount of hCG the test is designed to detect but remember that the time your hCG is increasing can be incredibly slow, so it's not an issue if your tests don't get darker straight away, plus they can sometimes get lighter if you happen to pick up another test which isn't as sensitive.

Also, once you've reached the amount of hCG the test detects that's all you'll get. A darker line doesn't mean you have a much higher number, it is more likely to be based on things like the amount of pigment in the ink or the diluteness of your urine. You'll normally find that the test line is lighter than the control line even when you know you have hCG levels above the threshold for the test.

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