Pregnancy Due Date Calculator

What Is a Pregnancy Due Date Calculator?

Your estimated due date is 40 weeks from the first day of your last menstrual period — that's the standard calculation used by the NHS, midwives, and your dating scan. But most pregnancies don't arrive exactly on that date. A full-term pregnancy is considered anywhere from 37 to 42 weeks, so the due date is best understood as the midpoint of a 5-week window.

The dating scan at 8–14 weeks is the most accurate predictor of when your baby will arrive. If the scan date differs from the LMP-based calculation by more than 7–10 days, your midwife will typically adjust the expected due date accordingly. The LMP method assumes a 28-day cycle with ovulation on day 14 — if your cycle is longer or shorter, ovulation (and conception) occurred at a different point.

Enter the first day of your last period and your average cycle length. You'll see your estimated due date, current gestational age in weeks and days, and key milestone dates: first trimester end, anatomy scan window, and estimated viability.

How Do You Use This Pregnancy Due Date Calculator?

Enter the first day of your last menstrual period. Click Calculate to see your estimated due date, current gestational age, and trimester information.

1. Recall the first day of your last menstrual period.
2. Enter that date in the LMP date field.
3. Click the Calculate button to generate results.
4. Read your estimated due date in the output panel.
5. Check your current gestational age in weeks and days.
6. Review the trimester milestones table below for upcoming developments.

How Does the Pregnancy Due Date Calculator Formula Work?

The formula used: Due Date = Last Menstrual Period + 280 days (Naegele's Rule)

What Are Some Example Calculations?

If your last period started on 1 January 2026, your estimated due date is 8 October 2026. At 8 weeks, you would be in the first trimester.

When Should You Use a Pregnancy Due Date Calculator?

As soon as you have a positive test — to work out how far along you are and when your midwife booking appointment and dating scan should be. In England, the booking appointment is ideally before 10 weeks. The 12-week scan is offered between 11 and 14 weeks. Knowing your dates means you can request these at the right time.

When sharing news with employers. UK statutory maternity leave requires you to notify your employer at least 15 weeks before your due date. The calculator gives you that notification deadline alongside your due date, so you're not caught out by the employment law timing.

What Do These Terms Mean?

How Do the Options Compare?

What Are the Best Tips to Know?

• Record the first day of each period in a phone app so the LMP date is always available.
• Book a dating ultrasound between weeks 8 and 14 for a more precise due date.
• Share the calculator result with your midwife at the booking appointment for cross-reference.
• Remember that only 5% of babies arrive on the exact due date — plan for a two-week window.
• Adjust the calculation if your cycle is longer than 28 days: add the extra days to the due date.
• Start taking 400 micrograms of folic acid daily as soon as pregnancy is confirmed.

What Mistakes Should You Avoid?

• Entering the date of a positive test rather than the first day of the last menstrual period.
• Assuming the due date is exact — it is an estimate with a two-week margin.
• Using this calculator with an irregular cycle without noting that the result may be less accurate.
• Confusing gestational age (from LMP) with fetal age (from conception, roughly 2 weeks less).