Last menstrual period Conception date Milestone timeline

Pregnancy Due Date Calculator

Find your estimated due date, how many weeks pregnant you are today, your current trimester, and a full pregnancy milestone timeline — all calculated instantly in your browser.

Medical disclaimer: This calculator provides estimates only and is not a substitute for professional medical advice. Always consult your healthcare provider or midwife for guidance specific to your pregnancy.
Due Date — Estimated delivery date
Weeks Pregnant — Gestational age today
Trimester — Current pregnancy stage
Calculate Due Date FAQ

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Calculate by Last Menstrual Period

The most common method. Enter the first day of your last period and your average cycle length.

Formula (Naegele's Rule): Due Date = LMP + (252 + cycle length) days. For a standard 28-day cycle this equals LMP + 280 days (40 weeks).

Results

Estimated due date—
Estimated conception date—
Last menstrual period—
Gestational age today—
Current trimester—
Viability milestone (week 24)—

Understanding Pregnancy Dating

How Naegele's Rule works

The standard formula for estimating a due date is Naegele's Rule, developed in the 19th century. It works by adding 280 days (40 weeks) to the first day of your last menstrual period.

The logic: average cycle is 28 days, ovulation occurs around day 14, and fetal development takes about 38 weeks from conception. Adding 14 days to conception gives 280 days from LMP.

  • Standard formula: LMP + 280 days (28-day cycle)
  • Adjusted formula: LMP + (252 + cycle length) days
  • Conception method: Conception date + 266 days

Trimesters overview

Pregnancy is divided into three trimesters, each representing a distinct phase of development:

  • 1st trimester (weeks 1–13): All major organs begin forming. Morning sickness is common. Risk of miscarriage is highest.
  • 2nd trimester (weeks 14–27): Baby movement begins. The anatomy scan (around week 20) checks fetal development.
  • 3rd trimester (weeks 28–40+): Rapid growth. Baby gets into position. Full term starts at week 37.

What affects due date accuracy

Several factors can make the LMP-based estimate less accurate:

  • Irregular cycles: If periods vary significantly, ovulation timing is harder to predict.
  • Cycle length: Cycles shorter or longer than 28 days shift ovulation — this calculator adjusts for this.
  • Recent contraception: Hormonal methods can delay return of regular ovulation.
  • Uncertain LMP: Remembering the exact date can be difficult.

Early ultrasound confirmation

An early ultrasound (typically at 8–12 weeks) is the most reliable way to confirm gestational age. It measures the size of the embryo or fetus directly.

If the ultrasound date differs from the LMP estimate by more than a week, healthcare providers will typically adjust the due date to match the ultrasound measurement.

  • Most accurate window: 8–12 weeks gestational age
  • Crown-rump length (CRL) is the primary measurement
  • Later ultrasounds are less accurate for dating

Age Calculator

Calculate exact age in years, months, and days. Find total days lived and days until the next birthday.

Calculate age →

Date Duration Calculator

Find the exact number of days, weeks, and months between any two dates. Useful for tracking time spans.

Calculate duration →
Important: The results provided by this calculator are estimates based on statistical averages. They are not a diagnosis or medical advice. Pregnancy dating can vary significantly based on individual factors. Always work with a qualified healthcare provider — such as an OB-GYN, midwife, or family physician — for accurate pregnancy assessment and care.

Frequently Asked Questions

How is a pregnancy due date calculated?

The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle differs from 28 days, the formula adjusts to LMP + (252 + cycle length) days. If you know your conception date, add 266 days (38 weeks). These are estimates — only about 5% of babies are born on their exact due date.

How accurate is a due date calculator?

A due date based on LMP is an estimate. Most babies are born within two weeks of their due date, but only about 5% arrive on the exact date. The most accurate way to confirm a due date is an early ultrasound between 8 and 12 weeks of pregnancy, which measures the baby's size directly.

What is Naegele's Rule?

Naegele's Rule is a standard obstetric formula for estimating a due date. Developed by German obstetrician Franz Karl Naegele, it states: take the first day of the last menstrual period, add 1 year, subtract 3 months, and add 7 days — which is equivalent to adding 280 days. It assumes a 28-day cycle with ovulation on day 14.

What if my cycle is not 28 days?

This calculator adjusts for non-standard cycle lengths. Instead of the fixed 280 days, it adds (252 + your cycle length) days to your LMP. For a 32-day cycle, that is 284 days. For a 24-day cycle, it is 276 days. This accounts for the shift in ovulation timing relative to a standard 28-day cycle.

What is gestational age?

Gestational age measures how far along a pregnancy is, counted in weeks from the first day of the last menstrual period — not from the date of conception. At the moment of conception, gestational age is typically already 2 weeks. Healthcare providers use gestational age as the standard measurement. A full-term pregnancy is 40 weeks gestational age (38 weeks from conception).