Pregnancy Due Date Calculator

Estimate your due date, current gestational week and trimester from your last menstrual period.

Your Details

days

Your Results

Estimated Due Date
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Trimester
1st Trimester
Week 0, Day 0
Current Gestational Age
0 days
Days Remaining
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Estimated Conception Date

40-Week Pregnancy Timeline

1st Trimester
2nd Trimester
3rd Trimester
Week 0 Week 13 Week 27 Week 40
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How Your Due Date is Calculated

This calculator uses Naegele's Rule, adjusted for your average cycle length, which is the standard method doctors use for an initial due date estimate.

Due Date = LMP + 280 days + (Cycle Length − 28)

Why dating starts from your last period, not conception

Pregnancy is dated from the first day of your last menstrual period (LMP) because that date is usually clear and memorable, even though conception typically occurs about two weeks later — around ovulation. This is why a pregnancy is described as "40 weeks" despite the baby only being conceived around week 2.

Why cycle length matters

Naegele's Rule assumes a standard 28-day cycle with ovulation around day 14. If your average cycle is longer or shorter than 28 days, ovulation — and therefore conception — likely happens correspondingly later or earlier, so this calculator adjusts your estimated due date to match your actual cycle length.

How to estimate your due date, step by step

  1. Enter the first day of your Last Menstrual Period (LMP).
  2. Enter your average menstrual Cycle Length in days, if it differs from the standard 28 days.
  3. Click Calculate to see your estimated due date, current week of pregnancy, and trimester.

Trimesters

How accurate is this estimate?

Only around 5% of babies are born exactly on their estimated due date — most arrive within a couple of weeks either side, which is completely normal. Your doctor may refine this estimate further using an early ultrasound scan, which measures the baby's actual size and is generally considered more accurate than LMP-based dating, especially if your cycles are irregular.

Disclaimer: This tool provides a general estimate only and is not medical advice. Please consult your doctor or gynaecologist for accurate dating and personalised prenatal care.

Frequently Asked Questions

The exact date of conception is rarely known with certainty, but the first day of the last menstrual period (LMP) is usually a clear, memorable date. Dating from LMP gives doctors a consistent, practical starting point, even though conception typically occurs about two weeks later, around ovulation — this is why a pregnancy is described as "40 weeks" despite the baby only being conceived around week 2.

Naegele's Rule assumes a standard 28-day cycle with ovulation around day 14. If your average cycle is longer or shorter than 28 days, ovulation (and therefore conception) likely happens correspondingly later or earlier than day 14 — so adjusting the calculation for your actual cycle length gives a more personalised estimate of conception and due dates.

Only around 5% of babies are born exactly on their estimated due date — most arrive within a couple of weeks either side, which is completely normal. LMP-based dating is a standard first estimate, but it assumes regular cycles and accurate recall of the LMP date, both of which can introduce error.

An early ultrasound measures the baby's actual size, which correlates closely with gestational age regardless of cycle irregularities or LMP recall accuracy. This makes it generally more reliable than LMP-based dating, especially for people with irregular cycles or uncertainty about their last period's start date — many doctors use ultrasound measurements to confirm or adjust the LMP-based estimate.

The three trimesters divide the standard 40-week pregnancy timeline into roughly equal stages of foetal development: 1st Trimester covers weeks 1-13, 2nd Trimester covers weeks 14-27, and 3rd Trimester covers weeks 28-40. Each trimester is associated with different developmental milestones and typical maternal experiences.

It doesn't account for irregular ovulation, medically assisted conception (like IVF, which uses a different dating method), or individual variation in cycle patterns beyond average length. This tool provides a general estimate only and is not medical advice — please consult your doctor or gynaecologist for accurate dating and personalised prenatal care.
Written and reviewed by Last updated