Disclaimer: I'm not a medical professional. I share well-researched insights and cite my sources for each post, but if you have a specific diagnostic medical question, you should seek the care of your physician.
If you go on any women's health forum or discussion board, you'll see a near constant stream of similar stories anxiously recounting recent unprotected sexual encounters that left either or both parties questioning: "When do I need to start worrying?" or really, "When should I take a pregnancy test?"
In the more nerve-wracking and dire of circumstances, people will be significantly late to get their periods and wonder - "Should I even believe the results of that (negative) pregnancy test?"
A couple of thoughts and considerations on whether and when you should take a pregnancy test to give yourself piece of mind after a risky encounter:
#1: You can only get pregnant during a handful of days in your cycle.
Specifically, your one egg released per month can really only be fertilized in the moment (hours-ish) during which you ovulate - BUT - at the right time, sperm can live for up to 5 days in your cervical crypts.
While I was taught in high-school to think of sperm swimming up to meet the (patiently) waiting egg, in reality, those wiley spermies are often the ones "waiting" for an egg to be released. I like to think of it in baseball terms. When someone ejaculates directly into your vagina during the fertile window, it's like their sperm are creating "coverage" all throughout the outfield on a baseball field. They're really tenacious, and strategically placed to pounce and catch that fly ball (your ova) when it comes.
So, if your sexual partner ejaculates inside you five days before you ovulate, four days before you ovulate, three days before you ovulate, two days before you ovulate, one day before you ovulate, or just hours before you ovulate, you have a decent chance of getting pregnant.
If your partner ejaculates inside you more than 24 hours after you ovulate, it's likely not happening.
And even that 5-day window can be shrunk further, depending on the quality of your cervical mucus (CM). (See my Fertility Awareness Method guide for more info there.)
#2: It takes approximately 9 days for a fertilized egg to implant in your uterus and start secreting hCG.
Human Chorionic Gonadotropin, or hCG, is the hormone that shows up on urine pregnancy tests and is produced by the cells which will eventually form the placenta. hCG is only produced after successful implantation, which, as indicated by my header, takes a long 7-14 days after fertilization to occur.
What's happening for those 9 days? A ton! But, to boil it down, fertilization actually occurs all the way up in the fallopian tubes - those are like the outfield in our baseball metaphor. When a sperm catches a released ova, like a pop fly into left field, that fertilized ova turns into a blastocyst, which begins replicating its cells on the journey towards the infield (your uterus). During this time, something called the trophoblast develops to surround the fertilized egg, and later, it helps implant the blastocyst into the uterus, pull the egg inside the uterine wall, and direct blood to it. Those trophoblast cells are so important that they eventually turn into the placenta - but, before then, they begin producing hCG.
Pop Quiz: If you ovulate on day 14 and it takes between 7 and 12 days for implantation to occur, the earliest your HcG-detecting pregnancy test will turn positive is what day before your missed period?
That's right ladies and gents! It's day 21 - at the crazy earliest!
So, when you see those early pregnancy tests claiming they're accurate as soon as 5 days before your missed period, they're usually taking the average time to implantation of 9 days and the average luteal phase of 14 days and assuming that you know exactly when you ovulated.
#3: Question - Do you (really) know when you ovulated?
If you're using the Fertility Awareness Method or ovulation predictor kits, the answer might be close to a "yes," but for the vast majority of us, the answer is more likely "no."
While the luteal phase - the second half of your menstrual cycle, leading up to your period - is quite consistent for most people and typically lasts 12-14 days ('normal' can still range up to 16 days), the follicular phase is a totally different beast. Tempered by your Hypothalamus-Pituitary-Adrenal (HPA) axis, the release of the hormones that regulate ovulation timing can be quite temperamental. So, if you're reading this article because you're extremely stressed that your period is "late," allow yourself to think back a few weeks to what was happening in your life and question whether you ovulated "on time" or not.
Common ovulatory disruptors include stress, travel, and illness.
If you are unsure when you ovulated in your cycle, and thinking about when it would be appropriate to take a pregnancy test, I invite you to be conservative in your estimates. Remember the 14-16 day longer luteal phase and the 12 day longer implantation timing when factoring in. your decision to test.
#4: If You're on Hormonal Birth Control, You Likely Aren't Ovulating (or Pregnant)
While it's possible to get pregnant using any method of contraception - hormonal or not - hormonal birth control is quite effective when used properly (i.e., taken within a 2 hr. window each day). Indeed, it's so effective, because it essentially prevents pregnancy in three different ways:
Preventing ovulation by "tricking" your body into thinking you've already ovulated (Read how Hormonal Birth Control Works, Part 1)*
Changing the quality of your cervical mucus (CM) so it's drier and less fertile (i.e. less hospitable for sperm to travel or live in)*
Maintaining an ultra-thin uterine lining that a fertilized egg cannot implant successfully.*
Long-term pill users are likely to experience shorter and lighter periods over time, given their thinner uterine lining (factor #3 above).
But, if you're really late or your period goes missing all together on birth control, it could be reason to see your care provider about the potential other conditions - especially hypothalamic amenorrhea.
*Resources: