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The Pregnancy Due Date Calculator estimates a baby's expected date of delivery (EDD) using Naegele's Rule, the most widely used method in obstetric practice worldwide. Based on the date of the Last Menstrual Period (LMP) and the average length of the menstrual cycle, this calculator provides an estimated due date, current gestational week, trimester, and days remaining in the pregnancy.
Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812, states that the estimated due date is 280 days (40 weeks) from the first day of the last menstrual period — assuming a standard 28-day cycle. For women with longer or shorter cycles, an adjustment of ±1 day per day of cycle deviation from 28 is applied to improve accuracy. Modern ultrasound-based dating has largely supplanted LMP-based methods for precision, but Naegele's Rule remains the standard clinical starting point used by obstetricians and midwives globally.
Pregnancy is divided into three trimesters: the First Trimester (weeks 1–13), marked by major organogenesis and the highest miscarriage risk; the Second Trimester (weeks 14–27), often called the most comfortable phase; and the Third Trimester (weeks 28–40), in which the fetus gains most of its weight and prepares for delivery. Only about 4–5% of babies are born on their exact due date, but 80% are born within two weeks of it.
The calculator applies Naegele's Rule with a cycle-length correction. The standard formula assumes a 280-day gestation from the LMP:
$$EDD = LMP + 280 + (CycleLength - 28) \text{ days}$$
For a woman with a standard 28-day cycle, this simplifies to LMP + 280 days (40 weeks). If her cycle is 30 days, 2 extra days are added; if it is 26 days, 2 days are subtracted, since ovulation — and therefore conception — occurs earlier or later relative to the LMP.
Gestational age is calculated as the number of days elapsed since the LMP divided by 7:
$$\text{Gestational Age (weeks)} = \frac{\text{Days since LMP}}{7}$$
Trimester boundaries are: First (days 0–90), Second (days 91–188), Third (days 189–280). Days remaining is simply the total pregnancy length minus days elapsed.
The due date is an estimate based on an assumed 28-day cycle and ovulation on day 14. Actual delivery varies; most clinicians consider 37–42 weeks as the normal range. A result in the first trimester warrants early prenatal appointments and confirmation via ultrasound. If you are in the second or third trimester, regular prenatal visits, glucose screening, and fetal movement monitoring become progressively important. Always confirm gestational age with your healthcare provider using first-trimester ultrasound when possible, as it provides a more accurate EDD.
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6 weeks pregnant with a standard cycle. 238 days (34 weeks) remain until the estimated due date.
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The 35-day cycle adds 7 extra days to the total pregnancy length (287 days). The woman is just entering the second trimester.
Naegele's Rule is a reasonable estimate but has limitations. Studies show it may overestimate gestational length in women with shorter cycles and underestimate it for longer cycles. First-trimester ultrasound (crown-rump length measurement between 8–13 weeks) is considered the most accurate dating method and can refine the EDD to within ±5–7 days.
If you don't know your last menstrual period, early ultrasound is the best method for dating. Between 8 and 13 weeks, crown-rump length (CRL) measurement provides a highly accurate EDD. After 13 weeks, multiple biometric measurements (head circumference, femur length, abdominal circumference) are used but are less precise.
Gestational age is measured from the first day of the last menstrual period and is the standard used in clinical practice. Fetal age (also called embryonic age or conception age) is measured from the date of fertilization, which typically occurs about 2 weeks after the LMP. Gestational age is therefore approximately 2 weeks greater than fetal age.
A due date is an estimate that does not change based on events during pregnancy. What can change is the actual date of delivery. Severe illness, preterm labor, or medical interventions like induction may result in birth before the due date. The EDD calculated at the start of pregnancy remains the reference point for gestational age throughout.
Approximately 10% of pregnancies last beyond 42 weeks (post-term). Most obstetricians recommend increased monitoring after 40 weeks and may discuss inducing labor by 41–42 weeks, as post-term pregnancies carry a slightly higher risk of placental insufficiency, meconium aspiration, and fetal macrosomia. Decisions are made based on maternal and fetal health assessments.
For twin pregnancies, the same Naegele's Rule applies initially, but typical delivery is earlier — around 36–38 weeks for dichorionic twins and around 34–36 weeks for monochorionic twins. Obstetricians typically plan elective delivery before 38 weeks for twins due to higher risks of complications in late pregnancy. The EDD is used as a reference, not a target.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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