Pregnancy Due Date: How Due Dates Are Calculated and What to Expect
Learn how pregnancy due dates are calculated using Naegele rule, ultrasound dating, and conception dates. Understand due date accuracy and gestational age.
How Due Dates Are Calculated
An estimated due date (EDD) is the approximate date when a pregnancy is expected to reach full term at 40 weeks of gestation. The most common method is Naegele rule, which calculates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). This method assumes a regular 28-day menstrual cycle with ovulation on day 14.
It is important to understand that the due date is an estimate. Only about 4-5% of babies are born on their exact due date. Full-term pregnancy is considered between 37 weeks and 42 weeks, giving a five-week window during which a healthy birth can occur. The due date is best thought of as the midpoint of this window, not a precise prediction.
Ultrasound Dating
First-trimester ultrasound between 8 and 13 weeks provides the most accurate dating. The crown-rump length (CRL) measurement is used to estimate gestational age within 5-7 days accuracy. If ultrasound dating differs from LMP-based dating by more than 7 days, the ultrasound date is typically used. Second and third-trimester ultrasounds are less accurate for dating because fetal growth rates become more variable.
Swipe sideways to compare columns.
| Trimester | Weeks | Key Developments |
|---|---|---|
| First trimester | Weeks 1-13 | Conception, implantation, organ formation, heartbeat detectable |
| Second trimester | Weeks 14-27 | Fetal movement felt, anatomy scan at 18-22 weeks, viability threshold at 24 weeks |
| Third trimester | Weeks 28-40 | Rapid growth, lung maturation, baby positions for birth, full term at 39 weeks |
Dating by Conception or IVF
For pregnancies conceived through IVF, the due date is calculated from the embryo transfer date with precise adjustment for embryo age. Known conception dates (from ovulation tracking or IUI) are more accurate than LMP-based dating. In these cases, the due date is calculated as conception date plus 266 days (38 weeks), which is the true gestational age from fertilization rather than from LMP.
How accurate is a due date from a first-trimester ultrasound?
First-trimester ultrasound dating has an accuracy of approximately 5-7 days. This is the most accurate method available and is more reliable than LMP dating when cycles are irregular or the LMP date is uncertain. After the first trimester, dating accuracy decreases significantly.
What happens if I go past my due date?
Pregnancies that reach 41 weeks are considered post-term. At 41 weeks, additional monitoring is typically recommended. Induction of labor is commonly offered between 41 and 42 weeks to reduce risks associated with prolonged pregnancy. Very few pregnancies continue beyond 42 weeks.