Calculate your estimated due date (EDD) from your last menstrual period, conception date, or IVF transfer. Shows current pregnancy week, trimester, and key milestone dates.
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⚠️ Educational only. Not medical advice. Consult your doctor for clinical guidance.
The 40-week pregnancy calculation was standardised by German obstetrician Franz Karl Naegele in 1812 and is still in use today. Naegele was also the first to accurately describe the mechanism of normal childbirth. His due date formula has remained essentially unchanged for over 200 years because it remains the most practical clinical tool available outside of early ultrasound dating.
The estimated due date (EDD) is calculated as 280 days (40 weeks) from the first day of the last menstrual period, or 266 days from the estimated date of conception. This standard, known as Naegele's rule, has been used in obstetrics since the early 19th century and remains the foundation of pregnancy dating.
Naegele's rule calculates the EDD as: LMP date + 9 months + 7 days (equivalent to LMP + 280 days). This assumes a standard 28-day cycle with ovulation on day 14. For cycles that differ from 28 days, the formula is adjusted: EDD = LMP + 280 days + (cycle length - 28 days). A 35-day cycle, for example, shifts the EDD 7 days later than the standard calculation.
The EDD is a statistical midpoint, not a precise prediction. Studies show that only about 4 to 5 percent of babies are born exactly on their estimated due date. The normal delivery window spans from 37 weeks (full term) to 42 weeks (post-term). Most babies are born within 2 weeks either side of the EDD, with the peak delivery rate occurring between 39 and 41 weeks.
IVF pregnancies have more precise dating because the fertilisation date is known exactly. For a Day 3 embryo transfer, the EDD is calculated as transfer date + 263 days (266 days from fertilisation minus 3 days already developed). For a Day 5 blastocyst transfer, the EDD is transfer date + 261 days. IVF-dated pregnancies tend to have more accurate gestational age estimates than LMP-dated pregnancies.
First trimester ultrasound (before 14 weeks) is the most accurate method of pregnancy dating after IVF, more accurate than LMP-based calculation for women with irregular cycles. The crown-rump length of the embryo at 7 to 12 weeks predicts gestational age to within 5 to 7 days. Second trimester ultrasound is less precise (within 2 weeks). Dating established by early ultrasound should not be changed based on later scans.
An estimated due date has an inherent uncertainty of approximately plus or minus 2 weeks for LMP-based calculations. Only about 4 to 5 percent of babies are born on their actual estimated due date. First trimester ultrasound (before 14 weeks) improves accuracy to within 5 to 7 days. For IVF pregnancies, the fertilisation date is known precisely and dating is highly accurate. The EDD is best understood as the midpoint of a likely delivery window spanning from 38 to 42 weeks.
Deliveries from 37 weeks (37+0) to 41 weeks and 6 days (41+6) are within the normal range for full-term birth. Before 37 weeks is preterm. After 42 weeks is post-term, at which point most obstetric guidelines recommend induction of labour due to increasing placental insufficiency risk. A delivery 2 weeks before the EDD (at 38 weeks) is considered normal and healthy. A delivery 2 weeks after the EDD (at 42 weeks) is at the outer edge of the normal range.
It depends on the discrepancy. NICE guidelines recommend changing the EDD based on ultrasound if the difference is more than 5 days at an 8 to 10 week scan or more than 7 days at an 11 to 13+6 week scan. Smaller discrepancies do not usually warrant changing the EDD. Once established by first trimester ultrasound, the due date should not be revised based on later growth scans, which reflect fetal size rather than gestational age.
Gestational age is counted from the first day of the last menstrual period and is 2 weeks longer than fetal age (the time since actual fertilisation). When a doctor says a pregnancy is 12 weeks, they mean 12 weeks gestational age, which corresponds to approximately 10 weeks since conception. All standard pregnancy dates, due dates, and developmental milestones are expressed in gestational weeks from LMP, not from conception.