Birth & Delivery

How to calculate your due date

Holly Pigache  |   28 Apr 2023

Calculating your due date is one of those milestone moments

Having a due date up in (mega watt lights) helps you prepare and is an exciting countdown to welcome your baby into the world. But if you’ve been on a fertility journey – or just have irregular periods – working out how to calculate your due date can be tricky.

Sometimes referred to as the EDD (estimated date of delivery) or the EDC (estimated date of childbirth), the date your baby is expected to be born can be calculated in a number of ways.

At The Ribbon Box, we’ve collated four common ways to calculate when to expect your baby.  Read on to find out more.

Knowing how to work out your estimated date of delivery gives you an idea of when to expect to deliver your baby.  However, it’s important to remember that due date prediction isn’t an exact science; these are educated guesses based on typical menstrual cycles and gestational lengths.

If you think of a due date as an idea of when you’ll give birth, this will help reduce any concern you might have of not delivering on your expected date.

You might be wondering whether you calculate your baby’s due date differently if you conceived naturally versus getting pregnant from fertility treatment.  With IVF, you can work out your due date from the date of embryo transfer.

You can predict when your baby will be born using a variety of dates:

  • date of your last menstrual period
  • conception date
  • 3- or 5-day embryo transfer date with IVF
  • date of your first ultrasound (if you have an early ultrasound)

Let’s look at each method of calculating pregnancy due dates in turn.

First day of your last period

Typically, a pregnancy lasts around 40 weeks (or 38 weeks from conception) so you can predict your due date by counting on 280 days (9 months/40 weeks) from the first day of your last menstrual period (not from the first day of a missed period).  Taking a pregnancy test on the first day of your missed period and seeing a BFP will indicate you’re around four weeks pregnant.

This calculation assumes you have a regular menstrual cycle of 28 days, so it might not be suitable if you have irregular periods.  For every day your cycle length is shorter, your due date will be one day earlier.  For every day your cycle length is longer than 28 days, your due date will be a day later.

Conception date

If you know the likely date you conceived, calculate your due date by adding 266 days from the probable date of conception.  For women who track their ovulation, it will be a little easier to determine the date of conception but for many women, knowing the date they conceived can be difficult.  This is because sperm can live in a woman’s body for up to five days and an egg can survive for about 24 hours after being released.  This gives a six-day window each month in which pregnancy can occur.

IVF transfer date

Predicting your due date after successfully getting pregnant through IVF is a little more straightforward.  If your embryo transfer was on day three, count on 263 days and if your embryo transfer was on day five count on 261 days to predict your IVF due date.

Ultrasound scan

Not all women are offered an early ultrasound so this isn’t always a reliable way to estimate a pregnancy due date.  Some pregnancy clinics routinely perform early ultrasounds.  Others might only recommend an early ultrasound for pregnant women over 35 years old, with irregular periods, a history of miscarriage or pregnancy complications, or the due date can’t be predicted from your last menstrual period or a physical exam.

calculate your due date ultrasound

Can I time when I have my baby and plan my due date?

Maybe you’re hoping for a September baby so your child is one of the oldest in their class at school.  Perhaps you’re avoiding trying to conceive around March in the hopes you won’t have a December baby.  Whatever your reasons for planning conception, you might be wondering: can I plan my due date?

At The Ribbon Box, we know how difficult it can be to get pregnant and 1 in 8 couples struggle to conceive.  Using an ovulation predictor kit (such as pearl) can help you time trying to conceive with the days you’re most and least likely to get pregnant.

Even if you’re fortunate to get pregnant when you plan to, you’re unlikely to be able to plan the exact day, week or even the month your baby is due. Try to enjoy the slight unpredictability of when you’ll give birth and don’t put too much pressure on yourself or your partner to have “the perfect pregnancy” – it doesn’t exist.

My due date has changed – is this okay?

Yes.  Doctors may adjust their prediction of when your baby is due if your first ultrasound was in the second trimester or you have irregular periods and the initial estimated due date was wrong.  Other reasons your doctor may change your predicted due date include having an abnormal fundal height (the measurement from your pubic bone to the top of your uterus) or your alpha-fetoprotein (AFP), a protein made by the baby, are outside the typical range.

It’s important to remember a due date prediction is just that.  There’s no guarantee your baby will be born on the predicted date (in fact, only 5% of babies are born on their estimated due date – most babies are born two weeks either side).  So use a due date calculator to give you an idea of when your baby is due to be born, not as an exact date to welcome your baby into the world.

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