Pregnancy Calculator

Finding out you're pregnant brings a lot of questions, and one of the first is usually "when is the baby due?" Your due date helps you plan everything from telling your employer to booking time off, and it's the date your midwife will use to time your scans and check-ups.

This guide explains how a due date is worked out, why pregnancy is counted in a way that can seem odd at first, and what the key dates are. To find your own dates, use the pregnancy due date calculator.

How your due date is calculated

Pregnancy is counted from the first day of your last menstrual period (often shortened to LMP), not from the day you conceived. That's simply because most people know when their period started, but very few know exactly when they conceived.

From that date, a pregnancy is expected to last about 280 days, or 40 weeks. A quick way to estimate it by hand is known as Naegele's rule: take the first day of your last period, add one year, subtract three months and add seven days. For example, if your last period started on 1 January 2026, your estimated due date is 8 October 2026.

If your cycle isn't 28 days

The standard calculation assumes a 28-day cycle, where ovulation happens around day 14. If your cycles are usually longer, you're likely to have ovulated later, so your due date moves later by the same number of days. A regular 35-day cycle, for example, pushes the due date back by about a week. The calculator adjusts for this automatically.

If you know your conception date

If you know when you conceived, perhaps because of fertility treatment or ovulation tracking, the due date is about 266 days (38 weeks) after conception. That's the same date as the period-based method, because conception usually happens about two weeks after the start of your last period.

Why you're "2 weeks pregnant" before you conceive

Because pregnancy is counted from your last period, the first two weeks or so of "pregnancy" happen before you've actually conceived. So when a midwife says you're 6 weeks pregnant, the embryo has been developing for about 4 weeks. It's a convention that lets every pregnancy be dated in the same way.

The three trimesters

Pregnancy is divided into three stages, each with its own changes:

Many people find the second trimester the most comfortable, as early sickness and tiredness often ease.

Key dates in an NHS pregnancy

In England, NHS antenatal care usually includes these milestones:

• Booking appointment, ideally by 10 weeks, where your midwife takes your history and arranges tests.
• Dating scan, between about 10 and 14 weeks. This measures the baby and gives the most reliable due date.
• Anomaly scan, between about 18 and 21 weeks, which checks how the baby is developing.
• 37 weeks, when a pregnancy is considered full term.
• 40 weeks, your estimated due date.
• 41 to 42 weeks, when you'll be offered an induction if labour hasn't started.

The calculator lists these dates for your pregnancy.

How accurate is a due date?

A due date is an estimate, not a deadline. Only around 1 in 20 babies arrive on their due date. Most are born between 37 and 42 weeks, which is the normal range for a full-term birth.

Telling your employer and planning leave

If you're employed, you must tell your employer you're pregnant at least 15 weeks before the week your baby is due, along with when you want your maternity leave to start. Your due date is used to work this out. You may be entitled to up to 52 weeks of Statutory Maternity Leave and up to 39 weeks of Statutory Maternity Pay, depending on your employment and earnings.

Looking after yourself

While you're pregnant, the NHS recommends taking a daily folic acid supplement until 12 weeks and a vitamin D supplement throughout pregnancy, avoiding alcohol and smoking, and attending all your antenatal appointments. If you haven't yet, contact a midwife or your GP surgery to start your antenatal care.

How your baby grows through the trimesters

In the first trimester, the baby's major organs and body systems begin to form, and by 12 weeks the baby is fully formed, though very small. In the second trimester, the baby grows rapidly, and many people start to feel movements between about 16 and 24 weeks. In the third trimester, the baby gains weight, the lungs mature and, towards the end, the baby usually moves into a head-down position ready for birth. The NHS pregnancy pages describe what happens week by week.