Chances of getting pregnant calculator: your odds, day by day

How the chances of getting pregnant calculator works

You can only get pregnant in a short stretch of each cycle: the fertile window. It covers the five days before ovulation (sperm can survive that long) plus ovulation day itself, because the egg lives for only about a day. So the question that really matters is not which cycle day you had sex, but how far that day was from ovulation.

1. Enter the first day of your last period and your usual cycle length.
2. The calculator estimates ovulation about 14 days before your next period is due.
3. Pick the day you had sex. The calculator counts how many days before or after estimated ovulation it was.
4. It shows the matching probability from the Wilcox study, plus a small bar chart of the whole window so you can see where your day sits.

Chance of pregnancy by day: the Wilcox study

The figures come from a landmark study of 221 healthy women who were trying to get pregnant, published in the New England Journal of Medicine in 1995 by Allen Wilcox and colleagues. The women collected daily urine samples, so researchers could pin ovulation down from hormones and pick up pregnancies very early. Here is the estimated chance of conceiving from sex on one single day:

Two things surprise most people. First, the best odds are in the two days before ovulation and on the day itself, not after. Second, even on the best day, the chance is about one in three, not a sure thing. Across a whole year, the NHS says more than 8 out of 10 couples where the woman is under 40 conceive with regular sex every 2 to 3 days.

How many days after my period can I get pregnant?

It depends on your cycle length, not on the period itself. In a short cycle the fertile window can start before your period has even finished. Here is a rough guide, assuming a 5-day period:

That's why sex right after your period can lead to pregnancy, especially with short cycles, and why there's no day of the cycle that's guaranteed safe. Our guide on how many days after your period ovulation happens goes deeper.

Worked example

Your last period started on 1 March and your cycles are usually 30 days. Your next period is due around 31 March, so ovulation is estimated about 14 days earlier, around 17 March. If you had sex on 15 March, that's 2 days before estimated ovulation: about 27%. If it was 10 March, that's 7 days before: very low, because sperm rarely survive that long.

Why your real odds can differ

• Ovulation moves. The calculator assumes it falls 14 days before your period. Stress, illness or travel can shift it by several days in any cycle.
• Age matters. The study looked at healthy couples. Chances per cycle fall gradually from the early 30s and faster after about 35.
• Health and history. PCOS, thyroid conditions, endometriosis, sperm quality and recent hormonal contraception all change the picture.
• Some very early pregnancies end. Daily urine tests in the study caught pregnancies that sometimes ended before a period was even missed.

Ovulation also changes how you feel. Many people notice more energy and a social, confident mood in the days around it, then a dip once the luteal phase begins. Tracking both helps you spot your own pattern.

When to talk to a doctor

The CDC suggests seeing a health care provider if you're under 35 and haven't conceived after a year of trying, or after 6 months if you're 35 or older. Don't wait if your periods are irregular or absent, you've had endometriosis or pelvic infection, or you've had more than one miscarriage.