🤰 Pregnancy Details
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👶 Your Due Date
How is a Pregnancy Due Date Calculated?
The most common method for calculating a pregnancy due date is Naegele's Rule — add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day menstrual cycle and that ovulation occurs on day 14. Our calculator adjusts for different cycle lengths automatically.
Only about 5% of babies are born on their exact due date. Most births occur within two weeks before or after the estimated due date. The due date is best understood as the midpoint of a delivery window rather than an exact date.
Different Calculation Methods
- Last Menstrual Period (LMP) — Most common method. Add 280 days to the first day of your last period. Adjusted for cycle length.
- Conception Date — Add 266 days (38 weeks) to your conception date. More precise if you know the exact date.
- IVF Transfer — For Day 5 blastocyst transfer, add 261 days. For Day 3 transfer, add 263 days.
- Ultrasound Dating — The most accurate method, especially in the first trimester. Uses the measured size of the embryo or fetus.
The Three Trimesters
- First Trimester (Weeks 1-12) — Rapid development of all major organs. Morning sickness common. Critical period for fetal development.
- Second Trimester (Weeks 13-26) — Most comfortable trimester for many women. Baby begins moving. Gender can be determined.
- Third Trimester (Weeks 27-40) — Baby gains weight rapidly. Braxton Hicks contractions begin. Preparation for birth.
Due Date Calculator — How Pregnancy Due Dates Are Calculated
A pregnancy due date (also called estimated date of delivery or EDD) is calculated as 280 days (40 weeks) from the first day of the last menstrual period (LMP). This standard is based on Naegele's Rule, developed in the early 19th century and still used worldwide. The 40-week count starts from LMP rather than conception because conception typically occurs around day 14 of a 28-day cycle — two weeks after the period begins — and LMP is a more reliably known date than the actual day of conception.
How Due Date Calculators Work — LMP Method
The LMP method adds 280 days to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation on day 14. For cycles longer or shorter than 28 days, the due date shifts accordingly — a 35-day cycle means ovulation around day 21, adding 7 days to the standard calculation. Our calculator adjusts automatically for cycle length. Only about 4-5% of babies are born on their exact due date — the due date is a central estimate around which birth most commonly occurs between 38-42 weeks. Use our pregnancy weight gain calculator to track healthy weight gain throughout your pregnancy timeline.
| Pregnancy Stage | Weeks | Key Milestones | Standard Tests |
|---|---|---|---|
| First Trimester | Weeks 1–12 | Organ formation, heartbeat at ~6 weeks | Dating scan, blood tests, NIPT |
| Second Trimester | Weeks 13–26 | Movement felt, gender visible, rapid growth | Anatomy scan at 18-20 weeks |
| Third Trimester | Weeks 27–40 | Lung maturation, baby positions for birth | GBS test, weekly monitoring from week 36 |
Ultrasound Dating — When It Overrides LMP Calculation
First-trimester ultrasound is the most accurate method for confirming or adjusting the due date. Between weeks 7-13, the crown-rump length (CRL) — the measurement from the top of the baby's head to the bottom of the buttocks — predicts gestational age with ±5 day accuracy. If ultrasound dating differs from LMP calculation by more than 7 days in the first trimester or more than 10 days in the second trimester, doctors typically adjust the due date to match the ultrasound. Later ultrasounds are less accurate for dating because fetal size variation between individuals increases significantly after the first trimester.
Preterm and Post-Term Birth — Understanding the Ranges
While the due date represents 40 weeks, healthy births occur across a range from 37-42 weeks. Terminology varies: extremely preterm (under 28 weeks), very preterm (28-32 weeks), moderate to late preterm (32-37 weeks), early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks) and post-term (42+ weeks). Babies born at 37+ weeks are generally considered ready for birth from a lung maturation standpoint, though the optimal window for birth outcomes is 39-40 weeks. Induction of labour is typically offered at 41-42 weeks to reduce risks associated with very prolonged pregnancy.
| Classification | Weeks | Considerations |
|---|---|---|
| Very Preterm | Under 32 wks | NICU care required |
| Late Preterm | 32–36 wks | May need extra support |
| Full Term | 39–40 wks | Optimal birth window |
| Post-Term | 42+ wks | Induction usually offered |
Preparing for Your Due Date — Practical Planning
While only 5% of babies arrive on the exact due date, knowing the estimate allows practical preparation. Hospital bag preparation is typically recommended from week 35-36. Maternity leave often needs to be arranged 4-8 weeks in advance of the due date depending on employer and country. Birth plan discussions with your midwife or obstetrician work best around weeks 32-36. Baby gear, nursery setup and infant car seat installation should ideally be completed by week 37, when early term delivery is possible. Freezer meals and household preparations in weeks 36-38 reduce the practical burden during the newborn period. Our due date calculator not only shows your estimated due date but also calculates key milestone dates throughout your pregnancy so you can plan with precision.
Twin and Multiple Pregnancy Due Dates
Twin pregnancies have different expected delivery timelines than singleton pregnancies. Dichorionic twins (separate placentas, lower risk) are typically delivered around 37-38 weeks. Monochorionic twins (shared placenta, higher risk) are typically delivered at 36-37 weeks due to increased risks later in pregnancy. Triplets are typically delivered around 34-35 weeks. These earlier delivery targets reflect the increased risks of very prolonged multiple pregnancies including twin-to-twin transfusion syndrome and placental insufficiency. Multiple pregnancy due dates calculated from LMP are less reliable than ultrasound dating, which is typically performed early and frequently in multiple pregnancies. Track your twin pregnancy timeline with our calculator and use our period tracker to understand your cycle before conception.
Calculating Conception Date from Due Date
If you know your due date but want to estimate when conception occurred, subtract 266 days (38 weeks) from the due date — this gives the approximate conception date, which corresponds to ovulation in most pregnancies. Alternatively, subtract 280 days from the due date to get the estimated LMP date, then add 14 days to find the approximate ovulation and conception date. These are estimates — fertilisation can occur within 24 hours of ovulation but sperm can survive 3-5 days, creating a conception window rather than a precise date. DNA paternity testing is the only definitive way to confirm paternity when the conception window spans multiple possible dates. Understanding your cycle and ovulation timing improves the accuracy of both forward (LMP to due date) and backward (due date to conception) calculations. Our due date calculator gives you the estimated due date, conception date and key milestone weeks all in one calculation.
A pregnancy journal — noting your due date, first heartbeat appointment, anatomy scan date and key milestone weeks — creates a meaningful record of the pregnancy that many families treasure long after the birth. Whether this is your first pregnancy or a subsequent one, having precise date calculations for each trimester boundary, the viable birth window (from week 24), the early term window (week 37), full term (week 39) and the post-term threshold (week 42) helps you understand exactly where you are in the pregnancy journey and what comes next at each stage. Enter your LMP or known conception date above to get your full pregnancy timeline instantly.