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The Pregnancy Due Date Calculator estimates a baby's estimated due date (EDD) based on the first day of the mother's last menstrual period (LMP) and her average menstrual cycle length. This tool uses the standard obstetric formula — Naegele's rule — which has been used by obstetricians for over 200 years and remains the foundation of gestational age dating in clinical practice worldwide.
Knowing your estimated due date is essential for prenatal care planning. It determines when to schedule ultrasounds, prenatal lab tests, and specialist consultations. It establishes the timeline for sequential prenatal screenings (nuchal translucency at 11-14 weeks, anatomy scan at 18-20 weeks, glucose tolerance test at 24-28 weeks). It helps parents plan maternity and paternity leave, prepare the nursery, and arrange childcare.
The standard gestation length is 280 days (40 weeks) from the LMP, which includes approximately 2 weeks before ovulation and 38 weeks of actual fetal development. Only about 5% of babies are born on their exact due date; most are born within 2 weeks before or after. The EDD is therefore best understood as the center of a normal distribution of delivery dates, not a precise prediction.
The calculator also adjusts for non-standard cycle lengths. Women with cycles shorter or longer than 28 days typically ovulate at a different point in their cycle. Each day the cycle differs from 28 days shifts the estimated due date by one day in the corresponding direction.
Naegele's Rule (formulated by Franz Karl Naegele, 1812) calculates the estimated due date as:
$$EDD = LMP + 280 \text{ days}$$
or equivalently, starting from the LMP date, subtract 3 months and add 7 days (and 1 year). In formula form:
$$EDD = LMP + 9 \text{ months} + 7 \text{ days}$$
The 280-day (40-week) standard assumes a 28-day menstrual cycle. For non-standard cycles, the cycle length adjustment is applied:
$$EDD_{adjusted} = LMP + 280 + (L - 28) \text{ days}$$
where $$L$$ is the average cycle length in days. A woman with a 30-day cycle ovulates approximately 2 days later, shifting the EDD 2 days forward. A woman with a 25-day cycle ovulates approximately 3 days earlier, shifting the EDD 3 days backward.
Gestational age at any point is measured from the LMP:
$$GA = \left\lfloor \frac{T_{today} - T_{LMP}}{7 \times 86{,}400{,}000} \right\rfloor \text{ weeks}$$
Trimester boundaries by gestational week:
Note: First-trimester ultrasound measurement of the crown-rump length (CRL) is considered more accurate than LMP dating and may revise the EDD by up to 5-7 days.
The Estimated Due Date (EDD) is the projected delivery date 40 weeks from your LMP, adjusted for your cycle length. Remember this is an estimate — only about 5% of babies arrive on their exact due date, and a normal full-term pregnancy ranges from 37 to 42 weeks. The Current Gestational Week tells you how far along the pregnancy is today. The Trimester shows which of the three major pregnancy phases you are in. The Days Remaining countdown shows how many days until the EDD.
Always confirm your EDD with your healthcare provider. First-trimester ultrasound dating (measuring fetal crown-rump length) is the gold standard for establishing gestational age and may revise the LMP-based estimate. If your cycle is irregular, ultrasound dating is especially important for accurate EDD assignment.
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LMP January 12, 2026 with standard 28-day cycle gives EDD October 19, 2026. At March 12, 2026 the pregnancy is at week 8 of the first trimester.
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A 32-day cycle shifts the EDD 4 days later than the standard calculation. LMP December 1, 2025 gives EDD September 14, 2026. At March 12, 2026, week 14 places the pregnancy at the start of the second trimester.
Naegele's rule is a standard obstetric formula for estimating the due date, described by German obstetrician Franz Karl Naegele in 1812. The rule states: take the first day of the last menstrual period, subtract 3 months, add 7 days, and add 1 year. This is equivalent to adding 280 days (40 weeks) to the LMP date. It assumes a regular 28-day menstrual cycle with ovulation on day 14.
The exact date of conception is rarely known with certainty, as it occurs approximately 2 weeks after the LMP and sperm can survive in the fallopian tubes for several days. The LMP date is almost always known precisely. Historically, physicians used LMP dating before conception timing was fully understood, and the practice has been retained because it is practical and standardized. Modern obstetric dating may use ultrasound CRL measurement to refine the estimate.
LMP dating is accurate to within ±2 weeks for most women with regular cycles. It can be less accurate for women with irregular periods, those who conceived while breastfeeding, those who became pregnant soon after stopping hormonal contraception, or those uncertain of their LMP date. First-trimester ultrasound measuring crown-rump length (CRL) is accurate to within ±5-7 days and is preferred when available. Second-trimester ultrasound is less accurate (±2-3 weeks).
A term pregnancy is defined as delivery between 37 weeks 0 days and 42 weeks 0 days of gestation. Early term is 37-38 weeks 6 days; full term is 39-40 weeks 6 days; late term is 41 weeks; post-term is 42 weeks or beyond. Only about 5% of babies are born on the exact EDD. Spontaneous labor most commonly begins at 39-41 weeks.
Yes. The standard 28-day cycle assumes ovulation on approximately day 14. Women with longer cycles ovulate later (e.g., day 18 for a 32-day cycle), meaning conception occurred later relative to the LMP. This shifts the EDD forward. Women with shorter cycles ovulate earlier, shifting the EDD backward. The adjustment is 1 day per 1-day deviation from 28 days. This correction is especially important for women with cycles consistently shorter than 25 or longer than 31 days.
No. The sex of the baby is determined at fertilization by the sperm cell (X chromosome = female, Y chromosome = male) and cannot be calculated from dates. Fetal sex can be determined by chorionic villus sampling (CVS) at 10-13 weeks, amniocentesis at 15-20 weeks, or via cell-free DNA (cfDNA) testing from about 10 weeks. Ultrasound can typically identify fetal sex at the anatomy scan around 18-20 weeks.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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