About This Tool
Calculate your estimated pregnancy due date based on the first day of your last menstrual period (LMP). This calculator uses Naegele's rule, the same formula obstetricians have relied on for over 200 years, to project your expected delivery date. Enter your LMP date and average cycle length to see your due date, current gestational age in weeks and days, which trimester you are in, estimated conception date, and a full timeline of key pregnancy milestones. Results are instant, free, and require no signup. For informational purposes only. Not medical advice. Consult a healthcare provider for any pregnancy-related decisions. Actual due dates are confirmed by ultrasound measurements, not calendar estimates. Only about 5% of babies arrive on their calculated due date, so think of this as the center of a four-week delivery window rather than a fixed target.
How Naegele's Rule Works
Naegele's rule estimates your due date by adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. The formula was first published by German obstetrician Franz Naegele in the early 1800s and remains the primary method for establishing an estimated delivery date (EDD) before ultrasound confirmation.
If your cycle is shorter or longer than 28 days, the calculator adjusts accordingly. For example, a 30-day cycle pushes the estimated ovulation two days later, so the due date shifts forward by two days. A 26-day cycle means ovulation likely occurred two days earlier, moving the due date two days sooner. This adjustment makes the estimate more accurate for women with predictable but non-standard cycle lengths.
Understanding Gestational Age vs. Fetal Age
Gestational age counts from the first day of the last menstrual period, even though conception typically happens about two weeks later. This means a woman is considered "two weeks pregnant" before conception has even occurred. Medical professionals use gestational age because the LMP date is usually known with more certainty than the exact day of conception.
Fetal age (also called embryonic age) counts from the actual date of conception and is roughly two weeks less than gestational age. When your doctor says you are 12 weeks pregnant, that refers to gestational age. The embryo itself has been developing for approximately 10 weeks. All standard pregnancy references, milestone charts, and trimester divisions use gestational age.
Trimester Breakdown and What to Expect
The first trimester spans weeks 1 through 12. During this period, the embryo develops major organs, the heart begins beating around week 6, and early pregnancy symptoms like nausea and fatigue are most common. The risk of miscarriage decreases significantly after the first trimester.
The second trimester covers weeks 13 through 26. Many women report feeling their best during this period as early symptoms subside. The anatomy scan around week 20 provides a detailed look at fetal development. Fetal movement becomes noticeable, and the baby grows rapidly.
The third trimester runs from week 27 until delivery, typically around week 40. The baby gains weight quickly, lungs mature, and the body prepares for labor. Regular prenatal visits become more frequent during this period. A baby born after 37 weeks is considered early term, while 39 to 40 weeks is full term.
Key Pregnancy Milestones
Several important milestones occur during pregnancy that your healthcare provider will monitor:
- Week 6: The fetal heartbeat becomes detectable via transvaginal ultrasound, typically beating at 90-110 bpm.
- Week 12-13: First trimester screening (nuchal translucency scan) can assess certain chromosomal conditions.
- Week 20: The anatomy scan (mid-pregnancy ultrasound) checks all major organ systems and can often reveal the baby's sex.
- Week 24: The viability milestone. With modern neonatal care, babies born at 24 weeks have a significant chance of survival.
- Week 37: Early term. The baby's lungs are generally mature enough for breathing without assistance.
- Week 39-40: Full term. This is the ideal delivery window for planned births when medically appropriate.
Accuracy and Limitations
Calendar-based due date calculations are estimates with a standard margin of error. Studies show that only about 4-5% of babies are born on their exact due date. Roughly 80% of babies arrive within a two-week window around the EDD (between 38 and 42 weeks). First-time mothers tend to deliver slightly later on average compared to women who have given birth before.
Ultrasound dating in the first trimester (before 14 weeks) provides the most accurate estimate of gestational age, typically within 5-7 days. If ultrasound dating disagrees with the LMP-based date by more than 7 days, healthcare providers usually adjust the due date to match the ultrasound. Later ultrasounds are less reliable for dating because normal variation in fetal size increases as pregnancy progresses.
Frequently Asked Questions
How accurate is a due date calculator?
A due date based on the last menstrual period is an estimate with about a two-week margin of error. Only 4-5% of babies arrive on the exact calculated date. First-trimester ultrasound provides better accuracy (within 5-7 days). Your healthcare provider may adjust the due date based on ultrasound measurements at your first prenatal visit.
What if my cycles are irregular?
If your cycle length varies significantly from month to month, a calendar-based calculation is less reliable. In that case, your doctor will likely rely on first-trimester ultrasound measurements to establish the most accurate due date. You can enter your best estimate of average cycle length here, but treat the result as a rough guide rather than a firm date.
Why does gestational age start before conception?
Gestational age counts from the first day of the last menstrual period because most women know this date with certainty. Conception typically occurs about 14 days later, but the exact date is rarely known. Medical convention uses LMP-based dating so that healthcare providers and patients share a consistent reference point for tracking pregnancy progress.
When is a baby considered full term?
The American College of Obstetricians and Gynecologists (ACOG) defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37-38 weeks, late term is 41 weeks, and post-term is 42 weeks or later. These distinctions matter because outcomes improve when delivery occurs during the full-term window.
Can my due date change during pregnancy?
Yes. If an early ultrasound shows that fetal measurements differ from the LMP-based estimate by more than a week, your healthcare provider will typically adjust the due date. This is common and does not indicate a problem. Once a due date is confirmed by first-trimester ultrasound, it generally stays fixed for the rest of pregnancy.
What is Naegele's rule and why is it still used?
Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period. Despite being over 200 years old, it remains widely used because it provides a quick, reasonably accurate estimate that requires no special equipment. It serves as the starting point for prenatal care until ultrasound confirmation is available.