How Your Due Date Is Calculated and Why It Can Change During Pregnancy

Why your due date is an estimate, not a deadline

The term 'due date' can make it sound like a fixed appointment your baby has agreed to keep, but it's really an estimate of when your pregnancy reaches around 40 weeks. Most babies arrive in the weeks either side of this date rather than on it exactly, which is completely normal and expected.

Understanding how the estimate is built, and why it sometimes shifts, can take some of the pressure off. It helps to see your due date as a useful planning marker for antenatal care rather than a countdown clock.

How the first estimate is worked out

At your booking appointment, usually between 8 and 12 weeks, your midwife will ask about the date your last period started. This is one of the first steps in working out an early estimated due date, alongside questions about your general health, any previous pregnancies, and your wellbeing.

This early calculation gives a starting point, but it relies on regular cycles and an accurately remembered last period date, which isn't always straightforward. That's one reason a further check is built into routine antenatal care.

The dating scan and why numbers can shift

Around 11 to 14 weeks, you'll usually be offered an ultrasound scan often called the dating scan. As well as checking your baby's development and whether you're expecting more than one baby, this scan measures your baby to work out a more precise estimated due date.

Because this measurement is based on your baby's actual size rather than an assumption about your cycle, it can move your due date earlier or later than the original estimate from your last period. This is a routine, expected part of dating a pregnancy accurately, not a sign that anything is wrong.

If your dates change after the scan, your midwife will update your maternity notes and use the new estimate for the rest of your antenatal care and appointment planning.

What to expect as your pregnancy progresses

Antenatal care in the UK follows a general pattern of check-ups designed to support you and your baby, with the number of appointments depending on whether it's your first pregnancy. Your due date acts as a reference point for when certain scans, tests and discussions are offered, rather than a strict deadline.

If you reach your later appointments, around 38 and 40 weeks, and you're still pregnant, your midwife will talk through your choices for what happens next. This is a standard conversation built into routine care, not something to worry about ahead of time.

• Bring your maternity notes to every appointment, including your dating scan
• Ask your midwife to explain any change to your estimated due date so it's recorded clearly
• Note down the date of your last period before your booking appointment if you can
• Use your due date as a general guide when planning maternity leave or travel, allowing some flexibility

When to speak to your midwife or maternity team

It's normal to feel a little unsettled if your due date changes, especially if you'd already mentally planned around the first estimate. If you have questions about your specific dates, your measurements, or anything about your individual pregnancy timeline, your midwife, GP or maternity triage team are best placed to talk this through with you, since they can look at your full notes and circumstances.

For more general reading on what to expect at each stage of antenatal care, you can browse other guides on the Bump Circle journal at /blog, alongside this one, as you prepare for your appointments and think through your own pregnancy timeline.