Pregnancy Due Date Calculator — By Last Period
Last reviewed: April 2026
This pregnancy due date calculator uses Naegele's Rule — the standard method used by OB/GYN providers — to estimate your baby's due date from the first day of your last menstrual period (LMP). The standard calculation assumes a 28-day cycle; if your cycle is longer or shorter, this calculator adjusts the conception estimate and due date accordingly. The tool outputs your estimated due date, current gestational age (if already pregnant), estimated conception window, trimester end dates, and a visual milestone timeline with key developmental and clinical checkpoints. All calculations are date-only estimates — actual due dates are confirmed by early ultrasound, which remains the most accurate dating method. Use this tool for general planning reference only.
Calculate Your Estimated Due Date
🗓 Pregnancy Milestone Timeline
Helpful pregnancy tracking resources:
- What to Expect When You're Expecting — The classic week-by-week pregnancy guide
- Pregnancy Journal — Record milestones from your due-date timeline week by week
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How Due Dates Are Calculated
This calculator uses Naegele's Rule — the standard obstetric method for estimating a due date from the first day of your last menstrual period (LMP). The formula adds 280 days (40 weeks) to your LMP, then adjusts for cycle length relative to the standard 28-day cycle: a longer cycle shifts the due date forward; a shorter one shifts it back.
Gestational age is counted from the LMP — not from conception. This means that at the moment of conception (approximately 2 weeks after LMP in a typical cycle), you are already counted as 2 weeks pregnant by clinical convention. A full-term pregnancy spans 40 weeks (280 days) from LMP, though only about 4% of babies arrive on their exact calculated due date.
Your healthcare provider will establish or confirm your estimated due date at your first prenatal visit. First-trimester ultrasound — typically between 8 and 13 weeks — measures the embryo directly and provides a more accurate date than LMP-based calculation alone, particularly when cycles are irregular. This tool provides a reference estimate only — it does not replace clinical assessment.
Why Your Due Date Is an Estimate, Not a Deadline
The term "due date" implies a level of precision that does not reflect how birth timing actually works. Research shows that roughly 80% of babies are born within two weeks before or after the calculated date. A pregnancy completed anywhere from 37 weeks (early term) through 42 weeks is within the normal range of variation. Individual factors — including the onset of labor, fetal development, and the mother's health — all contribute to when delivery actually occurs.
The due date is clinically useful as a reference point for scheduling prenatal care, tracking fetal development milestones, and identifying pregnancies that may need monitoring if they extend past 41–42 weeks. It is not a target to be met, and a few days past the date is not inherently a sign of a problem. Your obstetric provider is the right resource for any questions about timing, fetal movement, or signs of labor.
Frequently Asked Questions
How accurate is a due date calculator?
A due date from LMP is a reference estimate. Only about 4% of babies are born on their exact due date; roughly 80% arrive within two weeks before or after. LMP-based calculations assume a 28-day cycle with ovulation on day 14, which varies from person to person. First-trimester ultrasound is more accurate because it measures embryo size directly against established growth standards. Your obstetric provider will use clinical assessment — including ultrasound findings — to establish or confirm your estimated due date.
What if my cycle is not 28 days?
This calculator adjusts the due date for cycle lengths other than 28 days. A longer cycle shifts the estimated due date forward by the extra days; a shorter cycle shifts it back. However, this is an approximation — it assumes you ovulate a consistent number of days before your next period, which is not always the case. Irregular cycles make LMP-based dating less reliable. First-trimester ultrasound is the standard method for accurate dating when cycles are unpredictable. Always confirm your due date with your healthcare provider.
When will I find out my due date for sure?
Your obstetric provider will establish or confirm your estimated due date at your first prenatal appointment. A dating ultrasound (typically 8–13 weeks) measures the embryo directly and is the most accurate method. If the ultrasound date differs from the LMP-based date by more than 5–7 days in the first trimester, the ultrasound date is generally used as the clinical due date. The tool on this page provides a reference estimate only — it does not replace your provider's clinical assessment.
How is gestational age counted?
Gestational age is counted in weeks from the first day of your last menstrual period — not from conception. At the time of conception (around 2 weeks after LMP in a typical cycle), you are already counted as 2 weeks pregnant by obstetric convention. A full-term pregnancy is 40 weeks from LMP. Babies born between 39 weeks 0 days and 40 weeks 6 days are considered full term; 37–38 weeks 6 days is early term; 41 weeks or beyond is late term.
Related Calculators
Sources & Methodology
Formula: Naegele's Rule — Estimated Due Date (EDD) = First day of last menstrual period (LMP) + 280 days (40 weeks), adjusted for cycle length relative to the standard 28-day cycle. Conception date estimated as LMP + 14 days (average ovulation day).
Source: Standard obstetric dating method; American College of Obstetricians and Gynecologists (ACOG) Committee Opinion on methods for estimating the due date. Note: First-trimester ultrasound dating is more accurate than LMP-based calculation, particularly when cycle length is irregular.
Last reviewed: April 2026