Pregnancy Due Date
Last reviewed: May 2026
The Naegele rule (used since the 1800s) estimates delivery at 280 days from the first day of your last menstrual period.[1] This assumes a textbook 28-day cycle, but many women have cycles ranging from 21 to 35+ days. Longer cycles typically mean later ovulation and a later actual due date. First-trimester ultrasound measurements provide the most accurate dating if there is a discrepancy.[2] Track your pregnancy week-by-week with the Pregnancy Week Calculator.
| Milestone | Week | Trimester |
|---|---|---|
| First heartbeat detectable | 6–7 | 1st |
| End of embryonic period | 10 | 1st |
| First trimester screening | 11–13 | 1st |
| Anatomy scan | 18–22 | 2nd |
| Viability milestone | 24 | 2nd |
| Glucose screening | 24–28 | 2nd–3rd |
| Full term | 39–40 | 3rd |
The estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14 — assumptions that do not hold for all women. A woman with a 35-day cycle likely ovulated around day 21, meaning the standard calculation overestimates gestational age by approximately one week. This calculator allows you to adjust for cycle length, providing a more accurate estimate than the standard 280-day rule.
| Calculation Method | Basis | Accuracy |
|---|---|---|
| LMP (Naegele's Rule) | First day of last period + 280 days | ±2 weeks (assumes 28-day cycle) |
| Adjusted LMP | LMP + (cycle length − 28) days + 280 | ±1–2 weeks |
| First trimester ultrasound | Crown-rump length measurement | ±5–7 days |
| Conception date (if known) | Conception + 266 days | ±1 week |
| IVF transfer date | Transfer + 266 − embryo age | ±3–5 days |
Pregnancy is dated from the first day of the last menstrual period — not from conception. Since ovulation typically occurs around day 14 of the cycle, the actual gestational period from conception to birth is approximately 266 days (38 weeks), not 280 days. This dating convention exists because the LMP date is usually known with certainty, while the exact conception date is often uncertain. The result is that during "week 1" and "week 2" of pregnancy, the woman is not yet pregnant — ovulation and conception occur around week 2–3 of the gestational calendar.
Full term is defined as 39 weeks 0 days through 40 weeks 6 days. Early term is 37–38 weeks, late term is 41 weeks, and post-term begins at 42 weeks. Only about 4% of babies arrive on their exact due date. Approximately 80% are born within two weeks of the EDD — 10 days before to 10 days after. First-time mothers tend to deliver slightly later than the due date on average, while subsequent pregnancies often deliver a few days earlier.
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First | 1–12 | Implantation, heartbeat (6w), major organs form, first ultrasound (8–12w) |
| Second | 13–27 | Gender visible (18–20w), quickening (16–22w), anatomy scan (20w), viability (24w) |
| Third | 28–40 | Lung maturity (34–36w), full term (39w), engagement/lightening (36–38w) |
Several factors influence whether a baby arrives before, on, or after the due date. First pregnancies average 5 days past the EDD. Maternal age affects timing — women over 35 have slightly higher rates of both preterm and post-term delivery. Genetics play a significant role; if your mother or sisters delivered early or late, you are more likely to follow the same pattern. Carrying multiples shortens gestational length — the average twin pregnancy delivers at 36 weeks, and triplets at 32 weeks. Medical conditions including gestational diabetes, preeclampsia, and placenta previa can necessitate earlier delivery. Cervical length measured via ultrasound between 18–24 weeks is one of the strongest predictors of preterm birth risk.
Since the due date is an estimate with a 4-week window of normal delivery (37–41 weeks), practical planning should prepare for arrival as early as 37 weeks. Hospital bag preparation, nursery completion, work leave arrangements, and childcare plans for older children should all be finalized by week 36. Birth plan discussions with the healthcare provider typically happen between weeks 32–36. Car seat installation and inspection, pediatrician selection, and insurance updates for the new dependent are best completed by week 34 to avoid last-minute stress during the final weeks of pregnancy.
An ultrasound-based due date typically replaces the LMP-based date when the discrepancy exceeds 5–7 days in the first trimester or 10–14 days in the second trimester. First-trimester ultrasounds are the most accurate for dating because embryo growth is remarkably consistent in early pregnancy — a crown-rump length measurement at 8–12 weeks estimates gestational age within ±5 days. Later ultrasounds become progressively less accurate for dating because fetal growth rates diverge based on genetics, nutrition, and placental function. A due date established by first-trimester ultrasound should not be changed based on later scans.
Pregnancies extending beyond 41 weeks require increased monitoring because the risk of stillbirth, meconium aspiration, and macrosomia (large birth weight) increases after 41 weeks. Most obstetricians recommend induction of labor between 41 and 42 weeks. Non-stress tests and amniotic fluid assessments are typically performed twice weekly after 41 weeks to monitor fetal wellbeing. The ARRIVE trial demonstrated that elective induction at 39 weeks for low-risk first-time mothers did not increase cesarean rates and may reduce the risk of certain complications, though this remains a personal decision made in consultation with the healthcare provider.
Understanding ovulation timing helps both in calculating conception dates and in fertility planning. The fertile window spans approximately 6 days — the 5 days before ovulation plus the day of ovulation itself. Sperm can survive in the reproductive tract for up to 5 days, while the egg remains viable for only 12–24 hours after release. This means conception can result from intercourse that occurred several days before ovulation, which is why pinpointing the exact conception date is difficult without tracking ovulation through basal body temperature, cervical mucus changes, or ovulation predictor kits. For IVF pregnancies, the conception date is precisely known, making due date calculation significantly more accurate.
Most due date apps use the standard 280-day Naegele's calculation without adjusting for cycle length variation. This calculator accounts for your actual cycle length, which can shift the estimated due date by up to a week in either direction for women with cycles shorter than 25 days or longer than 32 days. A woman with a consistent 35-day cycle who uses a standard calculator receives a due date approximately one week too early, potentially leading to unnecessary anxiety about a "late" baby that is actually right on schedule for her specific physiology. Always discuss your cycle history with your healthcare provider to ensure the most accurate dating possible.
→ First-trimester ultrasound is the most accurate. Before 13 weeks, dating is within 5-7 days.[2]
→ Full term is 39-40 weeks. Earlier definitions of 37 weeks have been updated.[1]
→ Track weight gain. Use the Pregnancy Calculator for healthy weight ranges.
→ Only 4-5% deliver on the due date. Think of it as the center of a 4-week window.
See also: Pregnancy Calculator · Week Calculator · Ovulation · Age Calculator