How Is a Pregnancy Due Date Calculated?
A pregnancy due date — formally called the estimated date of delivery (EDD) — is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This standard, known as Naegele's rule, has been used in obstetric practice for over 200 years. It assumes a regular 28-day cycle with ovulation on day 14.
I built this calculator to give you a quick, reliable EDD estimate whether you are starting from your last period date or a known conception date. The 40-week count begins from the LMP, which means the first two weeks of your counted pregnancy occur before conception actually takes place — a quirk of how obstetric dating works.
LMP Method vs. Conception Date Method
Most people use the LMP method because the first day of the last period is usually a well-remembered date, while the exact day of conception often is not. If you do know your conception date — for example, from IVF or ovulation tracking — you can add 266 days (38 weeks) from that date instead, which accounts for the two-week offset.
- LMP method: add 280 days to the first day of your last period — the standard clinical approach
- Conception date method: add 266 days to the known date of conception
- IVF transfer: your clinic will calculate the EDD based on egg retrieval or transfer date
- Ultrasound dating: the most accurate method, especially in the first trimester (before 14 weeks)
What Happens at Each Trimester
Pregnancy is divided into three trimesters, each roughly 13 weeks long. Understanding where you are in the pregnancy timeline helps you anticipate key milestones and care appointments:
- First trimester (weeks 1–13): all major organs and structures form; nuchal translucency scan typically at weeks 11–13
- Second trimester (weeks 14–27): growth phase; anatomy scan typically at weeks 18–22; fetal movements begin
- Third trimester (weeks 28–40+): rapid weight gain; lung maturation; preparing for birth
Only about 5% of babies are born on their exact due date — most arrive within two weeks either side of the EDD. A birth between 39–40 weeks is considered full term. Between 37–38 weeks is early term; between 41–42 weeks is late term.
Frequently Asked Questions
How accurate is a due date calculated from LMP?
The LMP method is accurate to within about ±2 weeks for most people with regular cycles. It becomes less reliable with irregular cycles or if you are unsure of your LMP date. A first-trimester ultrasound is more accurate and can confirm or adjust your due date based on fetal measurements.
What if my cycles are longer or shorter than 28 days?
Naegele's rule assumes a 28-day cycle. If your cycle is regularly longer or shorter, your actual ovulation and conception likely occurred at a different point, shifting the true due date accordingly. Your healthcare provider will take this into account — early ultrasound dating is the most reliable way to establish your EDD in these cases.
When should I see a doctor after a positive pregnancy test?
Most guidelines recommend booking your first prenatal appointment as soon as you confirm a positive pregnancy test, ideally for a visit around 8–10 weeks of gestation. This is an estimate tool only — all pregnancy care decisions should be made in consultation with a qualified healthcare provider or midwife.