Due Date Calculator

What Is a Due Date Calculator?

That positive test changes everything — and the first question is always the same: when? The due date shapes every decision from here, from booking your first midwife appointment to planning maternity leave. NHS midwives use Naegele's Rule — your last menstrual period plus 280 days — and this calculator uses exactly the same method.

Enter either your last menstrual period (LMP) date or your conception date if you know it (common with IVF). The calculator shows your estimated due date, which week you're currently in, which trimester you're in, how many days remain, and a full milestone timeline from first heartbeat through to full term.

Only about 4% of babies arrive on their exact due date. Most arrive within two weeks either side. The NHS considers 37-42 weeks as the normal range for delivery, so treat the date as a strong estimate rather than a fixed appointment.

How Do You Use This Due Date Calculator?

Select whether you're entering your last menstrual period (LMP) date or conception date. Enter the date and click Calculate. You'll see the due date, current gestational week, trimester, days remaining, and a week-by-week milestone timeline.

1. Select your calculation method — Last Menstrual Period (LMP) or Conception Date.
2. Enter the date using the date picker.
3. Click Calculate Due Date to see the results.
4. Check your estimated due date and current gestational week.
5. Review which trimester you're in and how many days remain.
6. Scroll down to see the week-by-week milestone timeline.
7. Milestones already passed are highlighted in green with a tick.

How Does the Due Date Calculator Formula Work?

The formula used: Due Date = LMP + 280 days (Naegele's Rule). From conception date: Due Date = conception date + 266 days.

What Are Some Example Calculations?

LMP of 1 January 2026: Due date = 8 October 2026. At 10 weeks: First trimester. Days remaining: 210. Estimated conception: 15 January 2026.

When Should You Use a Due Date Calculator?

Use this as soon as you have a positive pregnancy test and know the date of your last period. It gives you an immediate estimate before your first midwife appointment. After your dating scan (usually around week 12), the NHS may adjust the due date based on the baby's measurements — but this calculator gives you a reliable starting point using the same formula your midwife uses.

What Do These Terms Mean?

What Are the Best Tips to Know?

• Your dating scan at around 12 weeks may adjust the due date by a few days — this is normal and more accurate than LMP alone.
• Only 4% of babies arrive on the exact due date. Most arrive within 2 weeks either side.
• The NHS considers 37-42 weeks as the normal delivery window — don't panic if your date passes.
• If you know the exact conception date (IVF, tracked ovulation), use that option for a more precise estimate.
• Book your first midwife appointment as soon as you have a positive test — don't wait for the due date.

What Mistakes Should You Avoid?

• Using the date of the positive test instead of the first day of the last period.
• Confusing gestational age (from LMP) with foetal age (from conception) — gestational age is always about 2 weeks ahead.
• Treating the due date as an exact delivery date rather than an estimate within a 5-week window.
• Not updating the due date after the dating scan if the midwife provides a revised estimate.