Pregnancy Due Date Calculator
Calculate your estimated due date, see your trimester breakdown, and track key pregnancy milestones — all from your last menstrual period.
Trimester milestones
Heart forms (week 5–6), limbs develop, brain grows rapidly. Morning sickness most common.
Baby can hear sounds, movements felt, sex can be determined. Most parents feel best now.
Baby gains weight rapidly, lungs mature, practises breathing. Delivery preparation begins.
How Is a Pregnancy Due Date Calculated?
The most common method for calculating a due date is Naegele's Rule, developed by German obstetrician Franz Karl Naegele in the early 19th century. The rule is simple: add 280 days (40 weeks) to the first day of the mother's last menstrual period. This accounts for approximately 2 weeks before conception (when the LMP occurs) plus 38 weeks of fetal development.
The reason doctors measure from the LMP rather than the conception date is practical — the exact date of ovulation and conception is rarely known, while the start of the last period is usually well-remembered or documented.
The formula
EDD = LMP + 280 + (cycle length − 28) days (adjusted for cycle length)
How accurate is the due date?
The estimated due date (EDD) is exactly that — an estimate. Research published in the American Journal of Obstetrics and Gynecology found that only 4–5% of babies are born on their exact due date. About 70% of births occur within 10 days of the EDD, and about 80% within 2 weeks. The wide window reflects natural variation in gestation length, which typically ranges from 37 to 42 weeks for healthy pregnancies.
Ultrasound dating, particularly when performed in the first trimester (before 13 weeks), is often more accurate than LMP-based calculation because it directly measures fetal size and development. Most healthcare providers use a combination of both methods.
Week-by-week key dates
| Week | Milestone | Significance |
|---|---|---|
| Week 4 | Positive pregnancy test | hCG detectable in urine |
| Week 6 | Heartbeat detectable | First ultrasound often scheduled |
| Week 12 | End of first trimester | Risk of miscarriage drops significantly |
| Week 20 | Anatomy scan | Detailed ultrasound, sex can be determined |
| Week 24 | Viability | Baby can survive with intensive medical care |
| Week 28 | Third trimester begins | Kick counts begin, GDM test |
| Week 37 | Early term | Baby considered full-term from week 39 |
| Week 40 | Due date | Expected delivery window |
| Week 42 | Post-term | Induction typically discussed |
Frequently Asked Questions
How is the due date calculated?
The standard calculation adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is called Naegele's rule and assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts accordingly.
How accurate is an estimated due date?
Only about 5% of babies are born on their exact due date. About 80% of births happen within two weeks either side. The term 'estimated due date' (EDD) is more accurate than 'due date' — it is a statistical midpoint, not a precise prediction.
What is a full-term pregnancy?
A full-term pregnancy lasts 39–40 weeks from the LMP. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond. Most healthcare providers consider delivery before 37 weeks to be premature.
What are the three trimesters?
The first trimester spans weeks 1–12. The second trimester covers weeks 13–26. The third trimester runs from week 27 to delivery. Each trimester brings distinct developmental milestones and maternal changes.
Can I use the calculator if I had IVF?
For IVF pregnancies, the due date is calculated differently. For a 5-day blastocyst transfer, add 261 days to the transfer date. For a 3-day embryo transfer, add 263 days. Your fertility clinic will provide a precise EDD based on your specific treatment protocol.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period — the standard medical measurement. Fetal age (conception age) is counted from the actual date of conception, typically about 2 weeks less than gestational age. Healthcare providers always use gestational age.