Pregnancy Due Date Calculator (LMP & Conception Methods)
Calculate your estimated due date and see key pregnancy milestones by trimester. Based on Naegele's Rule — the same method used by NHS midwives.
Pregnancy Due Date Guide
How Due Dates Are Calculated
The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculation adjusts accordingly. Only about 5% of babies are born on their exact due date — most arrive within 2 weeks either side. The NHS offers a dating scan at 8–14 weeks which provides the most accurate gestational age estimate.
Trimester Breakdown
First trimester (weeks 1–13): organ development, highest miscarriage risk, many women experience morning sickness. Nuchal translucency screening at 11–14 weeks. Second trimester (weeks 14–26): 'golden period' for many — energy returns, bump becomes visible, anomaly scan at 20 weeks. Third trimester (weeks 27–40): rapid growth, preparation for birth, 28-week growth scan, 36-week check for presentation. Term is defined as 37–42 weeks — under 37 weeks is premature, over 42 weeks is post-term.
Viability and Key Milestones
24 weeks: threshold of viability — survival outside the womb is possible with intensive neonatal care. 28 weeks: survival rate approximately 90%. 32 weeks: outcomes very close to full-term. 36 weeks: late preterm — babies often do well but may need some support. 37 weeks: early term. 39–40 weeks: full term — optimal outcome window. 42 weeks: post-term, usually induction is offered. The term 'weeks pregnant' counts from LMP, not conception — so at 4 weeks pregnant, the embryo is approximately 2 w
IVF Dating
For IVF pregnancies, the embryo transfer date determines gestational age. A 5-day blastocyst transfer adds 266 days (38 weeks) to the transfer date to calculate the EDD — equivalent to 19 days before the transfer being day 1 of the cycle. A 3-day transfer adds 268 days. IVF dating is generally more precise than LMP dating because the transfer date is exactly known. The first trimester dating scan is still recommended to confirm development.
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