Pregnancy Due Date Calculator

How does a due date calculator work?

A due date calculator takes the day your pregnancy started on the calendar and counts forward 40 weeks, or 280 days. The usual starting point is the first day of your last menstrual period (LMP). For example, if your last period started on January 1, 2026, your estimated due date is October 8, 2026.

For a long time, doctors used Naegele's rule: add one year, subtract three months and add seven days. However, months have different lengths, so the rule sometimes lands a day or two away from 280 days. That is why ACOG and most guidelines simply add 280 days. Our calculator does the same and shows the Naegele date next to it when the two differ.

If you like to check calendar math yourself, our date calculator counts the days between any two dates. Still, keep in mind that every due date is an estimate; your doctor or midwife confirms the final one.

Why do pregnancy weeks start before conception?

Pregnancy weeks run from the first day of your last period, not from conception. The reason is practical: few people know the day they ovulated, but most remember when their period started. As a result, with a 28 day cycle you are already counted as 2 weeks pregnant on the day of conception.

The calculator shows how far along you are in weeks and days. For instance, 12 weeks 3 days means 12 completed weeks plus 3 days, so you are in week 13 of your pregnancy. Your notes may write the same thing as 12w 3d or 12+3. In other words, 12+3 and week 13 describe the same stretch of time.

• Completed weeks: the 12 in 12+3.
• Current week: completed weeks plus one, so week 13.
• Trimester: first up to 13w 6d, second from 14w 0d to 27w 6d, third from 28w 0d.

For every method, the calculator counts your weeks back from the due date. That way an IVF date or a scan result fits into the same week count as a period date.

Does a longer or shorter cycle change your due date?

Yes. The 280 day rule assumes you ovulated on day 14 of your cycle. In practice, the phase after ovulation lasts about 14 days for most people, while the phase before it varies. So with a 35 day cycle, ovulation usually moves to around day 21.

The calculator adds or subtracts the difference: a 35 day cycle moves your due date 7 days later, and a 24 day cycle moves it 4 days earlier. For example, an LMP of January 1, 2026 with a 35 day cycle gives October 15, 2026 instead of October 8.

Use this adjustment only if your cycles are regular. If they vary a lot from month to month, or you conceived soon after stopping the pill, your period date can mislead. In that case an early ultrasound usually gives a more reliable date. Also note that the calculator accepts cycles from 21 to 45 days; anything outside that range is worth a conversation with your doctor.

How does an IVF due date calculator differ?

With IVF, your clinic knows the embryo's age and the transfer date exactly. For that reason, ACOG recommends dating pregnancies from assisted reproduction by the transfer date. For a day 5 blastocyst, the due date calculator adds 261 days to the transfer date; for a day 3 embryo, it adds 263 days.

The logic is simple. A 5 day old embryo is 5 days past fertilization, and fertilization sits at 2 weeks 0 days in the period based count. Therefore the transfer day equals day 19 of pregnancy, and 280 − 19 leaves 261.

If you know your conception date exactly, for instance from ovulation tests or a single intercourse date, choose the conception method, which adds 266 days. That said, conception is often only a guess, because sperm can survive for several days. If you are unsure, your period date or your scan gives a firmer starting point.

Can an ultrasound change your due date?

It can. In the first trimester, the crown rump length measurement shows gestational age within a few days. Your provider compares that result with the date from your last period, and if the gap passes a set limit, the scan date wins.

• up to 8w 6d: more than 5 days apart
• 9w 0d to 13w 6d: more than 7 days apart
• 14w 0d to 15w 6d: more than 7 days apart
• 16w 0d to 21w 6d: more than 10 days apart
• 22w 0d to 27w 6d: more than 14 days apart

These limits come from ACOG Committee Opinion 700. Another detail matters: once a first trimester scan sets your due date, later scans usually do not change it, because babies vary more in size as pregnancy goes on. In the UK, the NHS offers a 12 week scan that estimates more accurately how many weeks pregnant you are. Enter the scan date and a reading such as 8+2 into the ultrasound method, and your result matches the date on your report.

What percentage of babies are born on their due date?

Only a small share. In a 2013 study by Khambalia and colleagues that looked at 18,708 births, just 5% of babies arrived on their estimated date. About two in three were born within 7 days either side of it. The widely quoted figure of about 4 to 5% reflects the same pattern.

So think of your due date as a window, not a single day. Under the ACOG definitions, 37w 0d to 38w 6d is early term, 39w 0d to 40w 6d is full term and 41w 0d to 41w 6d is late term. From 42w 0d, the pregnancy counts as post-term.

The timeline in the calculator turns these limits into your own dates. For example, with a due date of October 8, 2026, term starts on September 17, 2026. In practice, that makes week 36 a sensible time to pack your hospital bag, and at 41 weeks your provider will talk you through the next steps.

When should you plan maternity leave and key appointments?

In the UK, GOV.UK says the earliest you can usually start maternity leave is 11 weeks before the expected week of childbirth. That week runs from Sunday to Saturday and contains your due date. Therefore the calculator finds the Sunday of your due week and counts back 11 weeks.

In the US, there is no national start date. Leave depends on your employer, on the Family and Medical Leave Act if you qualify, and on state programs. Still, the dates in the timeline help you plan the conversation with your employer early.

Other dates help as well. Around week 20, most people have the mid-pregnancy anatomy scan, and between weeks 24 and 28 comes glucose screening for gestational diabetes. The calendar button puts all of these into your phone. Once your baby is here, our age calculator tracks their age in days, weeks and months.

Should your clinic website offer a due date calculator?

If you run an OB/GYN practice, a fertility clinic or a midwifery service, yes. Nearly everyone who gets a positive test searches for this calculation in the first weeks, and they come back to the site they trust. However, Google holds health content to a high bar and looks for expert authorship, sources and a visible update date. We explain how in our E-E-A-T guide.

Privacy is the second point. Pregnancy details are health data, so a calculator should process them in the browser and never send them to a server. For the wider setup of your site, read our guide on building a GDPR compliant website.

Talha Aslan and the team build calculators like this one: fast, accessible and free of tracking. If you want one on your own site, take a look at our web design and SEO consulting services. You can also see our BMI calculator as another example of a health tool.