Pregnancy Due Date, Weeks, and Ovulation: What the Numbers Mean
A positive pregnancy test immediately raises a flood of date-related questions: When is the due date? How many weeks along am I? When was conception? When is the next fertile window? This guide answers all of those questions with the underlying formulas and context so you can interpret the numbers your care provider gives you.
How Due Dates Are Calculated
Most pregnancies are dated from the first day of the last menstrual period (LMP), not from conception. The standard method is Naegele's Rule:
- Add 1 year to the LMP date
- Subtract 3 months
- Add 7 days
This gives an estimated due date (EDD) 280 days (40 weeks) after the LMP. The Pregnancy Due Date Calculator applies Naegele's Rule automatically and also accepts a known conception date or ultrasound date as starting points.
Worked Example: Finding a Due Date
LMP = March 10, 2025.
- Add 1 year: March 10, 2026
- Subtract 3 months: December 10, 2025
- Add 7 days: December 17, 2025
Only about 4% of babies arrive exactly on their due date. A full-term birth is anywhere from 37 to 42 weeks of gestation.
Counting Pregnancy Weeks
Pregnancy is divided into three trimesters:
| Trimester | Weeks | Key milestones |
|---|---|---|
| First | 1-13 | Organ formation, highest miscarriage risk |
| Second | 14-27 | Movement felt, anatomy scan |
| Third | 28-40 | Rapid growth, birth preparation |
The Pregnancy Week Calculator tells you exactly which week and day you are in right now based on your LMP. The Pregnancy Weeks Remaining Calculator counts down how many weeks are left until your estimated due date - useful for planning leave, appointments, and preparation.
Ovulation and the Fertile Window
Ovulation typically occurs around day 14 of a standard 28-day cycle, but it can range from day 11 to day 21 depending on cycle length. Sperm can survive up to 5 days in the reproductive tract, so the fertile window is roughly the 5 days before ovulation plus the day of ovulation itself.
- Ovulation day estimate: cycle length - 14 days
- Fertile window: ovulation day minus 5 days to ovulation day
The Ovulation and Fertile Window Calculator takes your LMP and average cycle length and highlights your next fertile window on a calendar.
Common Mistakes
- Confusing conception date with LMP date: Conception occurs about 2 weeks after the LMP in a typical cycle. Most care providers date the pregnancy from the LMP, so at the moment of conception you are already considered 2 weeks pregnant.
- Assuming a 28-day cycle: Cycle lengths vary from 21 to 35 days. The ovulation calculators adjust for your personal cycle length.
- Treating the due date as exact: The EDD is a statistical midpoint. Plan for a range of 37-42 weeks.
FAQ
What if my cycle is irregular?
Irregular cycles make ovulation prediction harder. Tracking basal body temperature (BBT) or using ovulation predictor kits (OPKs) alongside the calculator gives more reliable results. An early ultrasound is the most accurate way to date the pregnancy.
Does the due date change after an ultrasound?
Yes. If the ultrasound measurements differ from LMP-based dates by more than a week in the first trimester (or more than 2 weeks later), care providers typically adjust the EDD to match the ultrasound measurement.
How many weeks is a full-term pregnancy?
The American College of Obstetricians and Gynecologists defines full term as 39 weeks 0 days to 40 weeks 6 days. Early term is 37-38 weeks; late term is 41 weeks; post-term is 42 weeks or later.