How the due date is estimated
This is Naegele's rule, the method midwives have used for two centuries. Pregnancy is dated from the first day of your last menstrual period (LMP) because that date is usually known, while the date of conception rarely is. Counting 280 days, exactly 40 weeks, from the LMP lands on the estimated due date. The cycle adjustment matters because ovulation happens roughly 14 days before the next period starts, not 14 days after the last one. If your cycles run long at 35 days, conception probably happened about a week later than the standard rule assumes, so the due date shifts a week later too. The "how far along" figure counts weeks and days from the adjusted start, which is how pregnancy is described at every appointment.
Worked examples
Trimesters and the weeks that matter
Pregnancy is split into three trimesters: weeks 1 to 12, weeks 13 to 26, and week 27 until birth. The calculator shows the dates your second and third trimesters begin. A few milestones are worth pencilling in around them. The NHS dating scan is usually offered between 10 and 14 weeks and produces the official due date, which replaces any calendar estimate, including this one. The anomaly scan follows between 18 and 21 weeks. From 37 weeks the baby is considered term, and if labour has not started by around 41 to 42 weeks your midwife will discuss induction. In the example above, the second trimester begins on 26 March 2026 and the third on 2 July 2026.
Why so few babies arrive on the day
Only around 1 in 20 babies is born on the estimated due date. The 280-day figure is an average across millions of pregnancies, and healthy pregnancies naturally vary by a couple of weeks in either direction, which is why "due month" is a more honest way to think about it. First babies also tend to arrive slightly later than average. None of this makes the estimate useless: it anchors every appointment, screening window and maternity leave plan, and it tells you which week you are in right now. It is simply a best guess until the scan refines it. Statutory maternity pay and leave planning also hang off the expected week of childbirth, so the estimate starts earning its keep well before the scan confirms it.
This tool is guidance, not medical advice. Your dating scan gives the official due date, and your midwife or GP is the right person for any questions about your pregnancy. The NHS pregnancy guide covers every week in detail. If bleeding, pain or reduced movements ever worry you, contact your midwife or maternity unit straight away rather than waiting for a routine appointment.
Frequently asked questions
How accurate is the estimated due date?
It is a starting point, not a promise. Only around 1 in 20 babies arrives on the due date itself, and most are born somewhere between 37 and 42 weeks. The dating scan, usually offered between 10 and 14 weeks, gives the official due date and replaces this estimate.
Why does it count from my last period rather than conception?
Most people know the date their last period started but not the date of conception, so pregnancy is dated from the first day of the last menstrual period. That means the standard 40 weeks includes roughly 2 weeks before conception actually happened.
What if my cycles are irregular or I am not sure of the date?
The estimate becomes less reliable, because the cycle adjustment assumes a regular pattern. Enter your best guess and treat the answer loosely. Your midwife will date the pregnancy from the scan measurement instead, which does not depend on your cycle at all.
What do the trimester dates mean?
Pregnancy is commonly split into three trimesters: weeks 1 to 12, weeks 13 to 26, and week 27 to birth. The calculator shows the dates the second and third trimesters begin, based on your adjusted cycle. Different sources draw the lines a week or so apart, so treat them as approximate.
Is my information stored?
No. Everything is calculated in your browser and nothing is sent to any server. Refreshing the page clears the dates.