Due Date Calculator – When Is My Baby Due?


Defaults to an average of 28 days; please adjust if required.

Defaults to an average of 14 days; please adjust if required.



So you’ve taken a pregnancy test and found out you’re pregnant. Congratulations!

Now you’re probably wondering what your baby’s due date might be.

You might have heard about a pregnancy calculator that helps you find out your due date.

BellyBelly’s pregnancy due date calculator gives you an estimated due date (EDD) of birth to mark on your calendar.

You don’t need to know the date of conception; all you need to know is the first day of your last period.

It’s important, though, to understand that calculating your due date is not an exact science.

The accuracy of a ‘due date’

First, you should bear in mind only 3-5% of babies are born on their estimated due date. So really it’s a guess – a ‘guestimate’.

Why are so few babies born on their due date?

To answer this, think of an apple tree. We know not all the apples are going to ripen on the same day – even though they come from the same ‘mother’ and the conditions of each ‘gestation’ have been quite similar.

The same applies to women and pregnancy. Each woman’s body is unique and everyone’s circumstances vary, which can affect the length of pregnancy and due date. This means gestational age will vary – not only from one woman to another but also from one pregnancy to the next.

Human gestation, or pregnancy, is classed as full-term once you have reached 39 weeks of pregnancy.

A pregnancy due date calculator will give you a result which is 280 days (40 weeks) since the first day of your last menstrual period (LMP).

This due date calculator method assumes you have an average menstrual cycle of 28 days, with ovulation occurring on day 14 (which is considered the conception date).

To calculate a due date based on this information isn’t 100% accurate. This is why so few babies are actually born on this exact date. They don’t know about a due date.

We also know first pregnancies are typically longer than subsequent ones.

The process of labor starts when baby is ready to be born and sends a signal to the mother’s body. The baby doesn’t know anything about dates or calendars!

You can read more in What Causes Labour To Start?

Ultrasound to calculate due date

Medicine is full of resources. Some women might be offered an ultrasound scan to confirm how many weeks pregnant they are. This helps to calculate an estimated due date.

Ultrasound for dating a pregnancy is usually performed if a woman has irregular cycles or is unsure about the date of her last menstrual period.

The early ultrasound is usually the most accurate for EDD. This is because the scan measures the baby’s size, which is very standard in the first few months.

You can read more in What Does AUA Mean On A Scan Result?

A date change doesn’t usually happen unless there is a big discrepancy between the due date given by the scan and the date calculation based on your last period.

If this happens, the medical practitioner usually goes by the scan date.

Menstrual cycle length

These calculating pregnancy tools are ‘one size fits all.

They take the first day of your last menstrual period (LMP) as day zero and assume your cycle is regular and lasts 28 days. Ovulation is assumed to have happened in the middle of your cycle and the date is usually considered as your conception date.

However, the date of conception isn’t necessarily the same as the date you ovulate.

An egg can live for up to 24 hours, so conception can happen a day after the egg’s release. You might also ovulate on a different day, and your menstrual cycle might be 23 or 31 days.

If you don’t know when the first day of your LMP was, and if you don’t track your fertile days, you might not be able to tell when you conceived.

A study in 2013 found that even when the date of ovulation was known, the length of pregnancy varied by 37 days.

Menstrual Cycle – Stages, Phases and What To Expect is packed with information about the menstrual cycle.

Due date calculator from conception

Sometimes you are sure of the date of conception, as it might have been the only time you had sex. In that case, a due date calculator will count this date as day 14, the last day of week 2.

If you’re certain about this, the numbers might not add up with your last period but your knowledge and certainty are much more valuable than a due date calculator.

Due date calculator IVF

For those who have achieved a pregnancy thanks to IVF, the egg retrieval, sperm collection and fertilization would have happened earlier.

When the date of the embryo transfer is known, that date should be taken as day 14, just between weeks 2 and 3.

If you conceived by artificial insemination, that date is also taken as day 14.

Ultrasound dating is the usual method when medical reproductive techniques are used.

Reverse due date calculator

A reverse due date calculator uses a method that’s the exact opposite of a due date calculator.

It works when you know your pregnancy due date and want to figure out when conception happened.

This might be because you want to know when you conceived but can’t remember your LMP.

Or perhaps you want to plan conception, and want to avoid having a baby at a certain time or year or month.

Trust your baby: he or she knows when it’s time to be born.

Unfortunately, no doctor – nor any due date calculator – will be able to give you a guaranteed magic date.

The vast majority of babies know when it’s time to be born, based on their own physical and developmental readiness.

Remember, your pregnancy is still considered full term until you reach 42 weeks.

Rest assured, if you do go post-dates (as frustrating as it might be), remind yourself that you are giving your baby the best chance to be born happy and healthy.

Of course, being patient can also mean fewer interventions and a better recovery for you and your baby.

Give your healthy, unborn baby the most precious gift of allowing him to arrive on his own birthday.

Check out our article, 40 Reasons To Give Your baby 40+ Weeks Of Pregnancy.

Due date calculator week by week

You can use our calculator to calculate your due date.

All you need to do is enter the first day of your LMP, then make any cycle adjustments (optional).

If you have a different cycle length, BellyBelly’s pregnancy calculator can take that into account.

You’ll notice making slight adjustments to the length of your cycle and luteal phase can affect the due date.

You’ll be able to discover the approximate date of your baby’s arrival, but keep fluctuations in mind.

Standard calculators or charts use averages – the same method a doctor or midwife would use. But remember, our bodies aren’t based on averages.

Once you have your due date worked out, be sure to sign up for BellyBelly’s pregnancy week by week emails – they’re awesome!

It’s like having a pregnancy calendar and journal all in one. You’ll know what to expect and what’s normal every week of your pregnancy. It gives you a good glimpse of your pregnancy milestones.

Be sure to speak to your health care provider so you can start getting some quality prenatal care with your choice of maternity care provider.

All the best for an amazing and healthy pregnancy!

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