Last updated: May 2026
Due date, current week, baby size, trimester, and full milestone timeline.
What date do you know?
From LMP (Naegele's Rule): Add 280 days (40 weeks) to the first day of your last menstrual period. For cycles other than 28 days, the formula adjusts: due date = LMP + 280 + (cycle length − 28) days.
From conception / ovulation: Conception typically occurs ~14 days after LMP. Add 266 days (38 weeks) to the conception date.
IVF 3-day transfer: Embryos are 3 days old at transfer. LMP equivalent = transfer date − 17 days (14 days to ovulation + 3 days of embryo age). Due date = LMP + 280.
IVF 5-day blastocyst transfer: LMP equivalent = transfer date − 19 days. Due date = LMP + 280.
Trimesters: First = Weeks 1–13 · Second = Weeks 14–27 · Third = Weeks 28–40.
Viability: Week 24 is the general threshold of viability — from this point, with intensive neonatal care, premature survival becomes possible.
⚠️ Due dates are estimates based on average cycle lengths. Only about 4% of babies are born on their exact due date — most arrive within 2 weeks either side. Always confirm dates with your midwife or OB-GYN and a dating ultrasound.
This calculator determines your estimated due date (EDD) and current gestational age using the two standard clinical methods.
Naegele's Rule is the standard obstetric method: add 280 days (40 weeks) to the first day of your last menstrual period. This accounts for the roughly 2-week gap between LMP and ovulation in a typical 28-day cycle.
Worked example: LMP of January 1, 2026 gives an EDD of October 8, 2026. At the time of calculation in mid-May 2026, gestational age would be approximately 19 weeks and 2 days - early in the second trimester.
Gestational age is measured from the LMP rather than from conception, so a pregnancy is considered two weeks along at the moment of conception. The three trimesters are: First (weeks 1-13), Second (weeks 14-27), and Third (weeks 28-40+). Full term is defined as 39-40 weeks.
The due date calculated from LMP assumes a regular 28-day cycle with ovulation on day 14. Women with irregular cycles may have a different actual conception date. First-trimester ultrasound (typically at 8-12 weeks) provides a more accurate due date by measuring the embryo's crown-rump length. Most OB-GYNs adjust the EDD based on ultrasound if there is a discrepancy of more than 5-7 days from the LMP calculation.
First trimester (weeks 1-13): Fetal heartbeat detectable around week 6; major organ systems form; miscarriage risk is highest during this period. Second trimester (weeks 14-27): Movement felt between weeks 16 and 22 (called quickening); anatomy scan at weeks 18-20; most miscarriage risk has passed. Third trimester (weeks 28-40+): Rapid fetal weight gain; Group B strep test at week 36; baby is considered full term at 39 weeks.
Gestational age is measured from the first day of the last menstrual period and is the standard clinical measurement. Fetal age (embryonic age) is measured from conception, which typically occurs about 2 weeks after LMP. When a doctor says you are 10 weeks pregnant they mean 10 weeks gestational age - the fetus is actually about 8 weeks old. This calculator uses gestational age, consistent with standard clinical practice.
The American College of Obstetricians and Gynecologists defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37 to 38 weeks 6 days; late term is 41 weeks; post-term is 42 weeks or beyond. Babies born at full term have the best outcomes for long-term health and development compared to those born even a few weeks early.
Pregnancy is divided into three trimesters spanning roughly 40 weeks from the last menstrual period. Each trimester is defined by distinct phases of fetal development and characteristic symptoms. Gestational age — the standard clinical measure — is counted from the LMP rather than from conception, which means the embryo is actually about two weeks younger than its gestational age at any point in the pregnancy.
Due dates are estimates, not deadlines. Fewer than 5% of babies are born on their calculated due date; most births occur within two weeks on either side. The 40-week estimate assumes a 28-day cycle and ovulation on day 14. First-trimester ultrasound, which measures crown-rump length, refines the estimate and is the most accurate way to date a pregnancy in the first half of the first trimester.
| Week | Trimester | Fetal Development | Common Symptoms |
|---|---|---|---|
| Week 4 | 1st | Implantation complete | Missed period |
| Week 8 | 1st | Heartbeat detectable | Nausea peaks |
| Week 12 | 1st | Major organs formed | Fatigue improves |
| Week 20 | 2nd | Anatomy scan | Movement felt |
| Week 28 | 3rd | Eyes open | Braxton Hicks begin |
| Week 37 | 3rd | Full term | Dropping/lightening |
| Week 40 | 3rd | Due date | Labor imminent |
How is a due date calculated?
The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day cycle and ovulation on day 14. For women with longer or shorter cycles, the estimated due date may be adjusted. First-trimester ultrasound is the most accurate dating method and is used to confirm or revise the LMP-based estimate.
What is Naegele's rule?
Naegele's rule, formulated by German obstetrician Franz Karl Naegele in the early 19th century, states that the estimated due date equals the first day of the last menstrual period plus 280 days (40 weeks). The common shorthand — add one year, subtract three months, add seven days — produces the same result for most months but can be off by a day or two depending on the month lengths involved. Adding 280 days directly is always precise.
How accurate is an estimated due date?
Only about 4–5% of babies are born on their exact due date. Roughly 80% of births occur within 2 weeks of the EDD. LMP-based calculation is accurate to within about 2 weeks for women with regular 28-day cycles. First-trimester ultrasound (8–12 weeks) is accurate to within 5–7 days and is the preferred method when LMP is uncertain or the cycle is irregular.
What counts as full term?
The American College of Obstetricians and Gynecologists defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37 to 38 weeks 6 days; late term is 41 weeks 0 days to 41 weeks 6 days; post-term is 42 weeks or beyond. Babies born at full term (39–40 weeks) have the best outcomes for respiratory health, feeding, and long-term development compared to those born even a week or two early.
What is the difference between gestational age and fetal age?
Gestational age is measured from the first day of the last menstrual period and is the universal clinical standard. Fetal age (also called embryonic age) is measured from conception, which typically occurs about 14 days after the LMP. A pregnancy described as "10 weeks gestational age" involves a fetus that is approximately 8 weeks old. This calculator uses gestational age throughout, consistent with all standard clinical references and prenatal care documentation.