Pregnancy Risk

I just had unprotected sex. What are my chances of getting pregnant?

While women could get pregnant at anytime, the chances of this happening vary considerably depending on where she is in relation to her period.

Let's take an example. A hypothetical "average" woman has a 28 day menstrual cycle. The first day of her period is considered "Day 1" of her menstrual cycle. She ovulates on day 14 of that cycle. She starts menstruating on day 23. During her menstruation (days 23-28), this woman has the virtually no chance of conception. Menstruation occurs when the egg released in the previous month's cycle does not get fertilized. Then the lining that would sustain a pregnancy is shed. No egg, no place to put the egg, no conception.

There isn't a chance of pregnancy until about day 10 or 11 of the next cycle--just before ovulation. This is because a woman's body becomes more hospitable to sperm, just before ovulation. And sperm can remain viable for two or three days inside a woman's body. So, even if ovulation occurs after sex, pregnancy may still happen. In fact, couples trying to conceive are advised to take advantage of the days just before the egg is released, since their chances are improved.

The chance of pregnancy peaks around day 13 or 14, just before or when the egg is released. Then the chance of pregnancy decreases for the next couple of days. After about day 16 or 17, the egg, if unfertilized is gone and so is the chance of pregnancy until the next cycle. All in all, this typical woman has about 5 fertile days out of 28, and only 2-3 with a high chance of pregnancy. These days are days 12-16.

But your actual mileage may vary. The reason women can get surprised by a pregnancy is because ovulation and menstruation aren't always so nicely linked in reality as in theory. An egg could be released at anytime. Few women have perfectly regular cycles. In addition, sometimes women mistake spotting for their periods, throwing off their estimations. Maybe you don't ovulate on day 14, maybe his sperm has a bit more longevity, maybe your egg has less "staying power".

Nevertheless, once the deed is done, this counting method is the best way to estimate one's risk of pregnancy. The 14 days to ovulation is fairly consistent among all women, even though the number of days from ovulation to menstruation varies considerably. Of course, it works better on women with regular cycles.

In any case, if your period does not arrive on schedule this month, wait a few days before panicking. Often, if a woman is worried about the possibility that she may be pregnant, this stress can play tricks with her hormones -- and delay the arrival of menstruation. Your body may be pulling a prank of sorts, but it doesn't know that your sense of humor may not be up to par about certain things.

REFERENCES
a listing of scientific articles and texts used.