Pregnancy Due Date Calculator

Calculate your estimated due date and see key pregnancy milestones based on your last menstrual period.

First Day of Last Period

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What Is a Pregnancy Due Date Calculator?

A pregnancy due date calculator adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method is called Naegele's Rule and is the standard used by midwives and obstetricians across the NHS.

The 280-day count assumes a 28-day menstrual cycle with ovulation on day 14. Only about 5% of babies arrive on the exact due date. Most births occur within a two-week window either side of the estimate. An early dating ultrasound at 8–14 weeks provides a more precise estimate.

Enter your LMP date below to see your due date, current gestational week, trimester status, and a week-by-week milestone timeline.

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.

Recall the first day of your last menstrual period.
Enter that date in the LMP date field.
Click the Calculate button to generate results.
Read your estimated due date in the output panel.
Check your current gestational age in weeks and days.
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)

Naegele's Rule calculates the due date as: Due Date = LMP + 280 days.

Start from the first day of the last menstrual period. Add 280 days (40 weeks). For example, an LMP of 1 January 2026 gives a due date of 8 October 2026 (1 Jan + 280 days).

An alternative shortcut: take the LMP date, subtract 3 months, and add 7 days. From 1 January: subtract 3 months = 1 October, add 7 days = 8 October. The result is the same.

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.

Standard 28-day cycle

LMP: 15 March 2026. Add 280 days. 15 March + 280 = 20 December 2026. Shortcut: 15 March - 3 months = 15 December + 7 days = 22 December.

Estimated due date: 20 December 2026. First trimester ends around 7 June 2026.

Early pregnancy check at 6 weeks

LMP: 1 February 2026. Today: 15 March 2026. Days since LMP: 42. Gestational age: 42 / 7 = 6 weeks exactly.

Gestational age: 6 weeks 0 days. Due date: 8 November 2026.

Late first trimester

LMP: 1 December 2025. Today: 26 February 2026. Days since LMP: 87. Gestational age: 87 / 7 = 12 weeks 3 days.

Gestational age: 12 weeks 3 days. Due date: 7 September 2026. Second trimester begins in 4 days.

When Should You Use a Pregnancy Due Date Calculator?

Use this calculator as soon as you get a positive pregnancy test. Knowing your estimated due date helps you book your first midwife appointment (recommended by week 10) and schedule the dating ultrasound scan (8–14 weeks).

Return to the calculator to track which trimester you are in and to review upcoming developmental milestones. Share the estimated due date with your midwife at your booking appointment — they will confirm or adjust it after the dating scan.

What Do These Terms Mean?

LMP

Last Menstrual Period. The first day of your most recent period, used as day 0 of gestational age.

Gestational age

The pregnancy duration counted from the LMP. Full term is 40 weeks (280 days).

Naegele's Rule

A formula adding 280 days to the LMP to estimate the due date. Named after German obstetrician Franz Naegele.

Trimester

One of three pregnancy phases. First: weeks 1–12. Second: weeks 13–27. Third: weeks 28–40.

Dating scan

An ultrasound between weeks 8 and 14 that measures the fetus to confirm or adjust the estimated due date.

Full term

A pregnancy between 39 weeks 0 days and 40 weeks 6 days. Births before 37 weeks are preterm.

How Do the Options Compare?

Week Range	Trimester	Key Milestones	Common Appointments
1–4	First	Implantation occurs. HCG hormone rises.	Home pregnancy test after missed period
5–8	First	Heartbeat detectable. Embryo reaches 1.6 cm.	Book midwife appointment (by week 10)
9–12	First	Organs forming. Fetus reaches 5.4 cm.	Dating ultrasound scan (8–14 weeks)
13–16	Second	Sex may be visible. Movement begins.	16-week midwife check
17–20	Second	Fetus reaches 25 cm. Mother feels kicks.	Anomaly scan (18–21 weeks)
21–27	Second	Lungs developing. Weight reaches 1 kg.	25-week midwife check (first pregnancy)
28–32	Third	Eyes open. Weight reaches 1.7 kg.	28-week blood tests and midwife check
33–36	Third	Head may engage. Weight reaches 2.7 kg.	34- and 36-week midwife checks
37–40	Third	Full term from week 39. Average birth weight 3.5 kg.	38- and 40-week checks. Induction discussed at 41 weeks.

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).

Frequently Asked Questions

How accurate is a due date calculator?

Due date calculators use Naegele's Rule, which assumes a 28-day cycle. Only about 5% of babies arrive on their exact due date. Most are born within two weeks either side.

When does each trimester start?

The first trimester runs from week 1 to week 12, the second trimester from week 13 to week 27, and the third trimester from week 28 to week 40.

Can I use this if I have an irregular cycle?

If your cycle is irregular, this calculator may be less accurate. An early ultrasound scan (usually at 8 to 14 weeks) provides a more reliable due date estimate.

What if I do not remember my last period date?

Request an early dating ultrasound scan. The sonographer measures the crown-rump length of the fetus to estimate gestational age. This scan is accurate to within 3–5 days when performed between weeks 8 and 14.

How does a longer menstrual cycle affect the due date?

Add the extra days to the estimated due date. A 35-day cycle means ovulation likely occurred on day 21 instead of day 14. Add 7 extra days to the standard 280-day calculation.

What is the difference between gestational age and fetal age?

Gestational age starts from the LMP (day 0). Fetal age starts from conception, which occurs roughly 14 days after the LMP. Fetal age is always about 2 weeks less than gestational age.

When is a baby considered full term?

A baby is full term from 39 weeks 0 days to 40 weeks 6 days. Early term runs from 37 to 38 weeks 6 days. Late term runs from 41 weeks 0 days to 41 weeks 6 days. Post-term starts at 42 weeks.

Will my due date change after an ultrasound?

Yes, if the dating scan estimate differs from the LMP calculation by more than 5–7 days. The midwife or obstetrician will adopt the ultrasound-based date as the official due date.

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