How Due Date is Calculated
The estimated due date (EDD) is the date when a pregnancy is expected to reach 40 weeks (280 days) of gestation. This is based on the assumption that pregnancy lasts 280 days from the first day of the last menstrual period (LMP), or 266 days from conception.
It is important to understand that the EDD is an estimate. Only about 4–5% of babies are born on their exact due date. The majority (approximately 80%) are born within two weeks of the due date, between 38 and 42 weeks of gestation.
Naegele's Rule
Naegele's Rule, named after German obstetrician Franz Naegele (1778–1851), is the standard method for calculating the due date from the LMP. The rule assumes a 28-day menstrual cycle with ovulation occurring on day 14.
(or equivalently: LMP + 1 year − 3 months + 7 days)
For women with cycles longer or shorter than 28 days, an adjustment is made:
All Calculation Methods
| Method | Formula | Accuracy |
|---|---|---|
| LMP (Naegele's) | LMP + 280 days (± cycle adjustment) | ±2–3 weeks |
| Conception Date | Conception + 266 days | ±1–2 weeks |
| IVF 3-day Transfer | Transfer + 263 days | ±3–5 days |
| IVF 5-day Transfer | Transfer + 261 days | ±3–5 days |
| First-trimester ultrasound | CRL measurement dating | ±5–7 days (most accurate) |
Pregnancy Timeline Diagram
Trimester Breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First Trimester | 1 – 12 | Organ formation, heartbeat detectable (~6 wk), highest miscarriage risk, morning sickness common |
| Second Trimester | 13 – 27 | Rapid growth, gender identifiable (~18 wk), fetal movements felt (~20 wk), anatomy scan |
| Third Trimester | 28 – 40 | Lung maturation, weight gain, head engagement, preparation for birth |
Accuracy of Dating Methods
The accuracy of pregnancy dating depends on the method used:
- LMP-based dating: Assumes regular 28-day cycles and ovulation on day 14. Accuracy decreases with irregular cycles. Error margin: ±2–3 weeks.
- First-trimester ultrasound (7–13 weeks): The most accurate clinical dating method. Crown-rump length (CRL) measurement is accurate to ±5–7 days. If ultrasound differs from LMP dating by more than 7 days, the ultrasound date is preferred.
- Conception date: More accurate than LMP because it eliminates variability in the follicular phase. Accuracy: ±1–2 weeks.
- IVF transfer: The most precise dating possible because the exact fertilization and transfer dates are known. Accuracy: ±3–5 days.
Key Pregnancy Milestones
| Week | Milestone | Details |
|---|---|---|
| 6 | Heartbeat Detectable | Fetal heartbeat visible on transvaginal ultrasound |
| 12 | End of First Trimester | Miscarriage risk drops significantly; nuchal translucency screening |
| 16 | Gender May Be Visible | External genitalia formed; may be seen on ultrasound |
| 20 | Anatomy Scan | Detailed ultrasound of all organs; halfway point |
| 24 | Viability Milestone | Survival outside womb possible with intensive NICU care (~50%) |
| 28 | Third Trimester Begins | Survival rate >90% if born; eyes open |
| 34 | Lung Maturity Approaching | Most organs functionally mature except lungs |
| 37 | Full Term | Baby considered full-term; safe for delivery |
| 40 | Due Date | Estimated date of delivery |
| 42 | Post-Term | Induction typically recommended; increased risk of complications |
Worked Example
A woman's last menstrual period was January 1, 2026, with a 28-day cycle:
If her cycle is 32 days (4 days longer than average):
Estimated conception date: LMP + 14 days (adjusted for cycle) = January 1 + 14 + 4 = January 19, 2026.
Frequently Asked Questions
How accurate is the due date?
The due date is an estimate, not a guarantee. Only about 4–5% of babies are born on their exact due date. Approximately 80% are born within 2 weeks of the EDD. First-trimester ultrasound provides the most accurate dating, within ±5–7 days.
Does cycle length matter?
Yes. Women with longer cycles ovulate later, so their actual gestational age is less than what a standard 28-day calculation would suggest. This calculator adjusts for cycle length by adding the difference between your cycle length and 28 days to the standard 280-day calculation.
What if my ultrasound date differs from my LMP date?
If a first-trimester ultrasound gives a due date that differs by more than 7 days from the LMP-based calculation, most providers will use the ultrasound date. In the second trimester, a difference of more than 10–14 days typically triggers a date change.
What does "gestational age" mean?
Gestational age counts from the first day of the last menstrual period, not from conception. This means the baby is about 2 weeks younger than the gestational age (since conception occurs around day 14 of the cycle). A "40-week pregnancy" is actually about 38 weeks of fetal development.
What is the difference between early and late term?
ACOG classifies term pregnancy as: Early term (37–38 weeks), Full term (39–40 weeks), Late term (41–41+6 weeks), and Post-term (42+ weeks). Elective delivery before 39 weeks is discouraged due to increased neonatal respiratory complications.