Pregnancy Conception Calculator

This calculator works backwards from a due date, the first day of your last period, or a dating ultrasound to estimate when conception happened. A due date of December 7, 2026 puts the most probable conception between March 14 and March 18, 2026, with intercourse between March 11 and March 18 the likely cause. All three methods land on the same answer because each one is really estimating the same thing: the first day of the last menstrual period.

Why the answer is a range and not a date

Conception cannot be pinned to a single day for most pregnancies, because two of the three variables involved are invisible. You know when your period started. You do not know exactly when you ovulated, and you do not know how long sperm waited before fertilising the egg. Both can move by several days.

Ovulation is far less predictable than the textbook day 14 suggests. A 2000 study in the BMJ following healthy women found that only about 30% had a fertile window falling entirely within the classic days 10 to 17, and timing varied even among women who described their cycles as regular. That variability is why this calculator reports a most probable window of five days and a wider possible window of eleven, rather than a single date.

Working from the last menstrual period

For a woman with regular cycles, conception usually falls 11 to 21 days after the first day of the last period. Ovulation typically happens about 14 days before the next period is due, so on a 28-day cycle that is day 14, and the egg is fertilisable for only 12 to 24 hours after release.

Notice what this means for cycle length. Ovulation counts backwards from the next period, not forwards from the last one. On a 35-day cycle, ovulation lands nearer day 21 than day 14, which shifts every window in this calculator by a week. That is why the last-period option asks for your average cycle length and the other two options do not need it.

This method struggles for anyone with irregular cycles, anyone who was recently on hormonal contraception, and anyone unsure of the date their last period began. In those cases an ultrasound is the more reliable route. To look forward rather than backward, the Ovulation Calculator predicts the fertile window from the same inputs.

Working from a due date

A due date is normally set by a healthcare provider and is itself calculated from the last period or an early scan. The standard method is Naegele's rule: 280 days, or 40 weeks, from the first day of the last menstrual period. Reversing it is straightforward, and it is what this calculator does when you choose the due date option, subtracting 280 days to recover the last period before working forward again.

Treat the due date as the centre of a distribution rather than an appointment. Only around 4% of babies arrive on their estimated due date, and the great majority arrive within two weeks either side. The Due Date Calculator runs the same arithmetic in the forward direction.

Working from an ultrasound

An obstetric ultrasound uses sound waves to image the fetus, and in early pregnancy it dates a pregnancy more accurately than anything else available. A first-trimester scan measuring crown-rump length is typically accurate to within about five to seven days. The same measurement taken in the third trimester can be off by three weeks or more, because growth rates diverge as pregnancy progresses.

This is why professional guidance leans on early scans for dating. Where an ultrasound before 9 weeks disagrees with the last-period estimate by more than about five days, obstetric practice is to redate the pregnancy from the scan. The permitted discrepancy widens as the pregnancy advances, reaching a couple of weeks in the third trimester. If you enter a scan here, use the gestational age the sonographer recorded on the day of the scan, in weeks and days, and the calculator subtracts it to recover the last period.

Pregnancies conceived through IVF are the exception to all of this. The date of fertilisation is known precisely, so dating comes from the transfer date and the embryo's age at transfer, not from a scan or a period.

Fertilisation, conception, and pregnancy

These three words get used interchangeably in ordinary speech, and they mean different things medically.

Fertilisation is the moment a sperm and an egg combine in the fallopian tube, producing a fertilised egg and starting embryonic development. Conception is the broader process of becoming pregnant, which includes fertilisation and implantation into the wall of the uterus. Implantation takes time: it occurs between 6 and 12 days after fertilisation, most commonly around days 8 to 10, and a meaningful share of fertilised eggs never implant at all.

The long-standing position in medicine, and in U.S. federal policy, is that pregnancy begins at implantation rather than at fertilisation. Some U.S. state-level definitions differ, treating fertilisation as the start, which is where much of the public confusion comes from. This calculator uses the medical definitions throughout.

The fertile window and why the intercourse dates run wider

Sperm survive far longer than eggs, and that asymmetry is the whole reason the intercourse windows on this page start earlier than the conception windows. Sperm can remain viable in the reproductive tract for up to five days in favourable cervical mucus. The egg lasts 12 to 24 hours after ovulation.

The practical result is a fertile window of about six days: the five days before ovulation plus ovulation day itself. Chances are not even across those days. The highest likelihood sits in the two days before ovulation, and it falls away sharply once the egg is released, which is the shape shown in the chart above. Intercourse the day after ovulation rarely results in pregnancy.

So a pregnancy dated to conception around March 16 could have started with intercourse as early as March 11, and this calculator reports both. The wider possible-intercourse range extends five days before the earliest possible conception date for the same reason.

Gestational age against fetal age

Pregnancy is counted from the first day of the last menstrual period, not from conception, which means the count starts about two weeks before you were pregnant at all. This is gestational age, and it is what your provider, your scan report, and every pregnancy app use.

Fetal age, sometimes called conception age or embryonic age, counts from fertilisation and runs roughly two weeks behind. A pregnancy at 12 weeks gestational age holds a fetus about 10 weeks past conception. Confusing the two is the most common reason people think their dates do not add up. The Pregnancy Calculator tracks gestational age week by week.

How this calculator works

Whichever method you pick, the first step is to recover the first day of the last menstrual period. A due date has 280 days subtracted. A scan has its recorded gestational age subtracted. A last period is used directly. Every window is then measured from that single anchor.

The most probable conception window runs from 12 to 16 days after the last period, centred on ovulation at day 14 and allowing two days either side. The possible conception window is wider, at 11 to 21 days, covering ovulation that arrives early or late. Most probable intercourse starts three days before the earliest probable conception date and runs to the end of that window; possible intercourse starts five days before the earliest possible conception date. Choosing a cycle length other than 28 days shifts every one of these by the difference, since ovulation tracks the end of the cycle.

Every figure here is an estimate built on population averages, and none of it substitutes for a dating scan or advice from a midwife or doctor. If the dates matter for a medical or legal reason, an early ultrasound is the measurement to rely on.