Baby Due Date
Calculate your baby's due date from LMP, conception, or IVF transfer date. Track pregnancy milestones.
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
Formula
Due Date = LMP + 280 days (or Conception + 266 days)
Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period, which assumes a 28-day cycle with ovulation on day 14. From a known conception or ovulation date the calculator adds 266 days instead. For IVF it counts 266 days from the notional fertilisation date, subtracting the embryo's age at transfer. An early ultrasound remains the most accurate way to date a pregnancy.
Worked Examples
Example 1: LMP Calculation
Problem:Last period started January 1, 2024.
Solution:LMP: Jan 1, 2024 Add 280 days (40 weeks) Due date: Oct 7, 2024 Gestational age on April 1, 2024: Jan 1 to Apr 1 = 31 + 29 + 31 = 91 days 91 / 7 = exactly 13 weeks 0 days 2024 is a leap year, so February contributes 29 days.
Result:Due October 7, 2024 (13 weeks 0 days on April 1)
Frequently Asked Questions
How is due date calculated?
From LMP: add 280 days (40 weeks). From conception: add 266 days (38 weeks). Only 5% of babies arrive on their due date. Normal range: 37-42 weeks.
What's the most accurate method?
Early ultrasound (8-12 weeks) is most accurate. IVF dates are very precise. LMP assumes 28-day cycle with ovulation on day 14, which varies.
When is the baby viable?
Around 24 weeks is commonly described as the threshold of viability, and survival rises steeply with each additional week of gestation. Actual survival at very early gestations depends heavily on the individual pregnancy and on the neonatal unit's capabilities, so published averages should not be applied to a specific pregnancy. Ask the obstetric or neonatal team for figures relevant to your care.
Why is pregnancy counted as 40 weeks?
It counts from the Last Menstrual Period (LMP), which is about 2 weeks before conception. So the first 2 weeks you aren't actually pregnant.
How often do due dates change?
A due date is redated when the ultrasound estimate differs from the LMP-based date by more than a set margin, and that margin widens as pregnancy advances: more than 5 days up to 8 weeks 6 days, more than 7 days from 9 to 15 weeks 6 days, more than 10 days from 16 to 21 weeks 6 days, and more than 14 days from 22 to 27 weeks 6 days. Your clinician makes this decision.
What is 'Full Term'?
ACOG defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks 0 days to 41 weeks 6 days, and post-term as 42 weeks 0 days and beyond. Elective delivery is generally not scheduled before 39 weeks without a medical reason.
Can I calculate due date if I have irregular periods?
LMP is less accurate for irregular cycles. An early ultrasound or known conception date is better. If using LMP, calculations assume a 28-day cycle.
What happens at 40 weeks?
You reach your due date! If labor hasn't started, you are monitored. Induction is usually discussed by 41-42 weeks.
Does IVF change the due date calculation?
Yes, IVF dates are precise because the exact fertilization/transfer time is known. We add 266 days to the fertilization date (minus embryo age).
What trimester am I in?
First trimester: from the last menstrual period through 13 weeks 6 days. Second trimester: 14 weeks 0 days through 27 weeks 6 days. Third trimester: 28 weeks 0 days until birth. Gestational age is counted from the first day of the last menstrual period, not from conception, which is why the count starts about two weeks before conception actually occurred.
Background & Theory
Pregnancy is traditionally divided into three trimesters, totaling roughly 40 weeks from the Last Menstrual Period (LMP).
Timeline:
- 1st Trimester (0 to 13 weeks 6 days): Rapid development, organ formation. High fatigue/nausea.
- 2nd Trimester (14 to 27 weeks 6 days): Growth phase, movement felt, anatomy scan. Usually higher energy.
- 3rd Trimester (28 weeks to birth): Weight gain, lung maturation, preparation for birth.
Baby Size:
We use fruit/veg comparisons because they are relatable. A "poppy seed" at 4 weeks grows to a "watermelon" by 40 weeks.