How to Calculate Your Pregnancy Due Date: Naegele's Rule Explained

How Pregnancy Due Dates Are Calculated

Your estimated due date (EDD) is calculated using Naegele's Rule, a method developed by German obstetrician Franz Karl Naegele in the early 19th century. The formula is simple: take the first day of your last menstrual period (LMP), add 280 days (40 weeks), and that is your estimated due date. The 280-day figure assumes a standard 28-day menstrual cycle where ovulation occurs on day 14. For cycles longer or shorter than 28 days, the due date is adjusted accordingly.

Why 40 Weeks?

Pregnancy is described as lasting 40 weeks, but conception typically occurs approximately 2 weeks after the last menstrual period. This means the actual gestational age — time since conception — is approximately 38 weeks, not 40. The 40-week count begins from the LMP because this date is more reliably known than the exact date of ovulation and conception. This is why pregnancy weeks are counted from the LMP rather than from the moment of fertilization.

Trimester Breakdown

Pregnancy is divided into three trimesters. The first trimester runs from week 1 through week 13 and is the period of most rapid development — the embryo develops all major organs during this time. This is also when morning sickness, fatigue, and other early pregnancy symptoms are most common. The second trimester runs from week 14 through week 27. Most women find this the most comfortable trimester — energy returns, morning sickness typically subsides, and the baby's movements become noticeable around week 18 to 22. The third trimester runs from week 28 through week 40 (delivery). The baby gains most of its weight during this period and moves into position for birth.

Key Prenatal Milestones

Several important appointments and screenings are scheduled at specific weeks. The first ultrasound is typically performed between weeks 8 and 14 and confirms the due date. The nuchal translucency scan for chromosomal abnormalities is performed between weeks 11 and 14. The anatomy scan (mid-pregnancy ultrasound) checks fetal development and can reveal the baby's sex at approximately week 18 to 20. The glucose tolerance test for gestational diabetes is typically performed between weeks 24 and 28. Group B streptococcus testing is performed between weeks 35 and 37. Weekly appointments begin around week 36 as delivery approaches.

How Accurate Is the Due Date?

Only approximately 5% of babies are born on their exact due date. The due date is an estimate — a target around which birth normally occurs. Most babies are born within 2 weeks before or after the estimated due date. A birth between 37 and 42 weeks is considered full term. Births before 37 weeks are considered premature. An early ultrasound — particularly between 8 and 13 weeks — provides the most accurate due date estimation and may adjust the LMP-based calculation if the measurements indicate a different gestational age.

How to Use Our Free Pregnancy Due Date Calculator

Our free pregnancy due date calculator at cookiescursor.com calculates your estimated due date from your last menstrual period date and average cycle length. Results show your due date, current week of pregnancy, trimester dates, and key milestone weeks. For informational purposes only — consult your healthcare provider for medical advice and accurate dating. No signup required.

Frequently Asked Questions

Can my due date change after an ultrasound?
Yes. If your first-trimester ultrasound measurements differ significantly from your LMP-based due date, your doctor may adjust the due date based on ultrasound findings, which are more accurate in early pregnancy.

What if my cycles are irregular?
Irregular cycles make LMP-based due date calculation less reliable. An early ultrasound is particularly important for accurately dating the pregnancy when cycles are irregular.

What does "full term" mean?
Full term is defined as 39 to 40 weeks of pregnancy. Early term is 37 to 38 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond.

Is a due date the same as a due week?
Many obstetricians prefer to give patients a "due window" of 2 weeks rather than a single date, to better reflect the natural variability of birth timing.

What happens if I go past my due date?
Most providers will monitor closely after 40 weeks and discuss induction options if you reach 41 to 42 weeks without going into labor naturally.

Can I calculate my due date from conception date?
Yes — add 266 days (38 weeks) to your known conception date to estimate your due date.

Calculate Your Due Date Now

Use our free pregnancy due date calculator for instant results. For informational purposes only. No signup required.