Pregnancy conception date calculator

Enter your last period, your due date or an ultrasound result. You will see your estimated conception date, the range it most likely falls in, the days of intercourse that could have led to it, and your due date. Your dates stay in your browser.

Your dates

First day of your last period Use the first day of your most recent period, the day full bleeding began.

Average cycle length Count from day 1 of one period to day 1 of the next. 28 days is a common starting point.

Dates stay in your browser. We never receive them.

Example: a 28 day cycle

Enter a date to see your own conception date

This is how a 28 day cycle works out, counting from the first day of the period. Your own dates replace this example as soon as you enter them.

Intercourse window, days 10 to 15
Conception, day 15
Implantation window, days 21 to 27
Most likely, days 23 to 25
Missed period, day 29

ConceptionCycle day 15

Intercourse that could have led to itCycle days 10 to 15

Due date40 weeks after day 1

Your estimated conception date Most likely between . A pregnancy is counted from the first day of the last period, so on the day you conceived you were already about two weeks pregnant.

Intercourse that could have led to it

Due date

Where you are today

Your key dates

First day of your last period
Intercourse that could have led to conception (up to five days before ovulation, and the day itself)
Estimated conception and ovulation
Implantation: most likely days (full window)
First day of a missed period, when a test is most reliable
Due date

Want more detail on the days after conception? Our implantation calculator and best time to conceive calculator take it further.

More pregnancy calculators

See all pregnancy calculators

How the conception date is worked out

1From your last period Day 1 + (cycle length minus 14) Ovulation tends to come about 14 days before your next period starts, and conception follows within a day. In a 28 day cycle that is 14 days after the first day of your period. A longer or shorter cycle moves the date by the same number of days.
2From your due date Due date minus 266 days A due date is 280 days after the first day of the last period, and conception comes 14 days after that, so it falls 266 days before the due date. That is about 38 weeks earlier.
3From an ultrasound Scan date minus age at scan, plus 14 A scan report gives the age of the pregnancy in weeks and days, counted the same way as a due date. Subtract it from the scan date to find day 0 of the pregnancy, then add 14 days. An early scan is the most accurate starting point.

What these dates can and cannot tell you

The conception date is a good guide to when to expect the milestones that follow. It is an estimate, because ovulation varies.

What the calculator uses

The first day of your last period and your usual cycle length, or a due date, or an ultrasound date and the pregnancy age it gave.
The standard rule that ovulation comes 14 days before the next period, which puts conception on day 15 of a 28 day cycle.
Sperm that survive up to five days and an egg that can be fertilized for about a day, for the intercourse window.

What it cannot tell you

Show the day you really conceived. Ovulation moves from one cycle to the next, so the true date can sit a few days either side.
Work well when your cycles are irregular or you are unsure of your last period. An early scan is a better starting point.
Say which act of intercourse led to the pregnancy when several fall in the window, or who the father is. A DNA test does that.
Tell you whether you are pregnant. A pregnancy test does that.

When did I conceive? Conception, ovulation and pregnancy age

Doctors count the age of a pregnancy from the first day of your last period, not from the day you conceived. That is why you are already about two weeks pregnant on the day an egg is fertilized: the first two weeks come before ovulation. In a 28 day cycle, conception falls about 14 days after your period began, and 266 days (38 weeks) before the due date.

Fertilization happens in the fallopian tube when a sperm meets an egg. An egg can be fertilized for only about 12 to 24 hours after ovulation, so fertilization and ovulation fall on the same day or a day apart. This calculator treats that day as the conception date. The fertilized egg then divides as it travels to the uterus and attaches 6 to 12 days after ovulation, which is implantation. Some definitions count conception from implantation, so a date you see elsewhere can differ by about a week.

Person holding a strip of printed ultrasound images in front of an ultrasound machine in a clinic — Ultrasound images from a scan. Photo by MART PRODUCTION on Pexels.

Sperm can survive in the reproductive tract for up to five days, so intercourse in the five days before ovulation, and on the day itself, can lead to a pregnancy. Intercourse after ovulation rarely does. In a study that tracked ovulation, the chance of conceiving was highest on the two days before ovulation and on ovulation day. The calculator uses the same days for its intercourse window.

If you are working out when you conceived because you want to count back from a test or a symptom, our conception calculator shows your most likely day with a window either side, and the pregnancy due date calculator shows how many weeks pregnant you are today.

Which starting point is the most accurate?

Starting point	How accurate it is	Best when
Early ultrasound (before 14 weeks)	The most accurate way to date a pregnancy. ACOG advises changing a period-based due date when an early scan differs by more than 5 days (up to 8 weeks 0 days) or more than 7 days (9 weeks 0 days to 13 weeks 6 days).	You have a dating scan report
Last period, regular cycles	Good when you are sure of the date. Ovulation still shifts by a few days around the estimate.	Cycles that are about the same length each month
Last period, irregular cycles	Can be a week or more out, because ovulation does not follow a fixed 14 days before your next period.	Use a scan instead if you have one
Due date	Only as accurate as the date it came from. A due date given at a first visit comes from your last period or from a scan.	You do not have the other dates

If your last period date and a scan disagree, go with the scan. Your care team will usually set the due date from an early scan and keep it for the rest of the pregnancy.

How to enter an ultrasound

Choose the Ultrasound tab, then enter the date of the scan and the age of the pregnancy it reported. Your report shows the age as weeks and days, for example 8 weeks 2 days or 8w2d. The calculator subtracts that age from the scan date to find day 0 of the pregnancy, then works out the conception date, the due date and where you are today.

If the scan gave you a due date, use the Due date tab instead. If your clinic changed your due date after the scan, enter the new date, because it already includes the change. Our pregnancy due date calculator can also show every milestone from that date.

If your cycles are irregular or you are unsure of your dates

A period-based estimate assumes you ovulated 14 days before your next period would have started. When cycles vary from month to month, that assumption is weaker and the real conception date can be a week or more away. Tracking ovulation with tests or temperature helps if you are still trying to conceive: our guide to calculating your ovulation day explains the options.

If you are already pregnant and unsure of your dates, a first-trimester scan gives the most reliable dating. Book a visit with your doctor or midwife and ask for a dating scan if you have not had one. Bleeding that is heavy, pain that is severe or any symptom that worries you needs a call to your care team straight away.

Common questions

Short answers about conception dates, ovulation and intercourse timing.

How do I calculate my conception date?

Pick the date you know. From your last period, add your cycle length minus 14 days to the first day of the period (14 days for a 28 day cycle). From a due date, subtract 266 days. From an ultrasound, subtract the pregnancy age on the report from the scan date and add 14 days. The calculator does the counting and shows a range of two days either side.

How many days after my period do you conceive?

In a 28 day cycle conception falls about 14 days after the first day of your period, on cycle day 15. It can happen anywhere from about day 11 to day 21 depending on your cycle length and when you ovulate. The calculator moves the date for cycles that are longer or shorter than 28 days.

How accurate is a conception date calculator?

It is an estimate. Ovulation can come a few days earlier or later than a formula predicts, so the most likely range is two days either side of the date shown. A scan in the first trimester is more accurate than a period-based date, especially if your cycles are irregular.

Can an ultrasound tell me the exact conception date?

No. A scan measures the baby and gives the age of the pregnancy within a few days. Subtracting 14 days from the start of that age gives a conception estimate that is as good as the scan, which is usually within about a week for an early scan.

What is the difference between conception and ovulation?

Ovulation is when an egg is released. Conception, or fertilization, is when a sperm meets it, which can only happen for about 12 to 24 hours after that. The two fall on the same day or one day apart, so this calculator treats them as the same date.

How long can sperm live inside the body?

Up to five days. That is why intercourse in the five days before ovulation, and on the day itself, can lead to a pregnancy. The intercourse window on the results page covers those days.

What if there were several days of intercourse in the window?

The calculator cannot tell which one led to the pregnancy. Any intercourse in the window could have, and the earlier dates need sperm that survived a few days. It also cannot show who the father is. Only a DNA test can.

Is conception the same as implantation?

No. Conception is fertilization. Implantation comes 6 to 12 days later, when the embryo attaches to the lining of the uterus. Some definitions count a pregnancy from implantation, so a date you see elsewhere can differ by about a week. Our implantation calculator shows that window.

Why is my due date different from the one my doctor gave me?

Doctors often change a period-based due date after an early scan. If your care team gave you a due date after a scan, enter that date or the scan details, because they already include the change.

What if my cycle is not 28 days?

Choose your usual length from 21 to 45 days. A 35 day cycle moves conception about a week later than a 28 day cycle, because ovulation comes later. If your cycles vary a lot, a scan gives a better estimate.

Can I use this if I conceived with fertility treatment?

This calculator is for natural conception. After treatment such as IUI or IVF your clinic dates the pregnancy from the procedure, so follow the due date it gives you.

Does the calculator save my dates?

No. They stay in your browser and are never sent to us. A session store in your own tab keeps them until you close the tab, so you can move between calculators.