When Did I Conceive?

When Did I Conceive? How to Estimate Your Conception Date

Why Is It Hard to Know When You Conceived?

Many people want to know exactly which day their pregnancy began. The difficulty is that doctors don’t date a pregnancy from conception. They count from the first day of your last menstrual period (LMP), because that date is usually easy to remember. The baby, however, is not conceived until after ovulation, when a sperm fertilises the egg. That typically happens well over a week after your period began.

Several things make the exact day hard to pin down:

• Ovulation isn’t always on the same day of the cycle.
• Sperm can live in the body for up to five days, so sex on one day can lead to conception days later.
• The egg can be fertilised for only about 12 to 24 hours after it is released.

For these reasons, a conception calculator gives you an estimated window rather than a certain answer.

Three Ways to Estimate Your Conception Date

1. From your last menstrual period

If your cycles are regular, conception usually falls around 11 to 21 days after the first day of your last period. This is a rough guide, because it is hard to know exactly when you ovulated. Irregular cycles, or not remembering the date your last period began, make the estimate less reliable.

Example: your last period began on 1 March. Conception most likely happened between about 12 and 22 March. Since sperm can survive for up to five days, sex from around 7 March to 22 March could have led to the pregnancy.

2. From your due date

If your doctor has given you an estimated due date, usually based on an ultrasound, you can work backwards. Conception is typically about 266 days before the due date.

Example: a due date of 5 July 2027 points to a conception date around 12 October 2026. As the due date itself is an estimate, allow a few days either side. If you only have your last period date, our due date calculator will give you the due date first.

3. From an ultrasound

An obstetric ultrasound uses sound waves to make live images of your baby. It is a standard part of prenatal care in many countries and shows how the pregnancy is progressing. Measurements can be taken from about five or six weeks after your last period, and they are most accurate early in pregnancy, becoming less precise later on.

When your periods are irregular or your dates are uncertain, an early scan is one of the most reliable ways to work out gestational age, and from that your likely conception window. Read why your due date can change after an ultrasound for more detail.

Comparing the Methods

Already know your conception date and want your due date instead? Use our conception calculator.

Fertilisation, Conception and Pregnancy: What Is the Difference?

People often use “fertilisation” and “conception” to mean the same thing, but in medical terms they are different steps.

• Fertilisation happens when a sperm and an egg join, usually in the fallopian tube. This creates a fertilised egg, which starts developing into an embryo.
• Implantation is when the developing embryo attaches to the lining of the womb, usually about six to twelve days after ovulation.
• Conception, in the medical sense, is the process of becoming pregnant, which involves both fertilisation and implantation.

Major medical bodies, such as the American College of Obstetricians and Gynecologists, consider a woman pregnant once implantation has taken place. Some laws and everyday usage define these terms differently depending on the country or state, so the medical definition may not match what you read elsewhere. For dating purposes, calculators use the estimated day of fertilisation as the conception date.

What to Do Once You Have Your Conception Window

1. Confirm your dates with your doctor. They may suggest an early dating scan.
2. Find your due date and how far along you are. Use our pregnancy calculator to see your weeks, trimester and milestones.
3. Keep a record. Write down your last period, your cycle length and any ovulation test dates for your appointments.
4. Learn how reliable the estimate is. Our guide on how accurate due date calculators are explains why dates are always estimates.

Frequently Asked Questions

Can I find the exact day I conceived?

Not usually. You can narrow it to a few days using ovulation tests, an early scan or your cycle dates, but the exact day cannot be known for certain.

How long after sex does conception happen?

It can take up to about five days, because sperm can survive in the body while waiting for the egg.

Is conception the same as the first day of pregnancy?

No. Pregnancy is dated from your last period, so you are already counted as about two weeks pregnant on the day you conceive.

Which is more accurate: the last period or an ultrasound?

An early ultrasound is generally more accurate, especially if your cycles are irregular or you don’t remember your last period date.