Pregnancy Due Date Calculator

How to Calculate Your Pregnancy Due Date

Your estimated due date (EDD) marks the end of a full-term pregnancy — 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). This calculation method, known as Naegele's Rule, has been used in obstetric practice since the 19th century and remains the standard recommended by the American College of Obstetricians and Gynecologists (ACOG).

The formula is simple: add 280 days (40 weeks) to the first day of your last period. In practice, clinicians often use the shortcut of adding one year, subtracting three months, and adding seven days to the LMP date. Both methods produce the same result.

It's worth noting that the 40-week standard assumes a 28-day cycle with ovulation on day 14. If your cycle is consistently longer or shorter, our calculator adjusts the estimate accordingly — a 35-day cycle shifts the due date approximately one week later than the standard calculation.

Why Is It Calculated From Your Last Period, Not Conception?

Most people find this counterintuitive. If pregnancy begins at conception, why does the clock start two weeks earlier — at the last period? The answer is practical: conception is difficult to pinpoint with certainty for most people, while the first day of a period is easy to remember and record. By using LMP as the universal starting point, clinicians have a consistent, reproducible reference that works across diverse cycle patterns.

In reality, conception typically occurs around day 14 of a 28-day cycle, roughly two weeks after LMP. This is why gestational age (counted from LMP) is approximately two weeks more than fetal age (counted from conception).

Three Ways to Calculate Your Due Date

Our calculator supports three methods, each suited to different situations:

• Last Menstrual Period (LMP): The most common method. Requires only the first day of your last period and your average cycle length. Accurate for cycles close to 28 days; our calculator adjusts for other cycle lengths.
• Conception date: If you know when you ovulated or conceived — for example, through ovulation tracking or IUI — you can calculate directly from conception. The EDD is 266 days (38 weeks) from conception.
• Ultrasound dating: If you've had an early ultrasound, your provider measured the embryo and assigned a gestational age. Ultrasound dating is most accurate in the first trimester (before 14 weeks) and can supersede LMP dating if there's a discrepancy of more than 7 days.

How Accurate Is a Due Date Calculator?

Your EDD is an estimate, not a deadline. Research consistently shows that only about 5% of babies are born on their calculated due date. Most babies arrive within two weeks before or after — the full-term window runs from 39 weeks to 40 weeks and 6 days, while "early term" spans 37–38 weeks and 6 days.

Several factors affect accuracy: irregular cycle length, conception timing uncertainty, embryo measurement variability at ultrasound, and normal biological variation in pregnancy length. Your OB-GYN may revise your EDD based on ultrasound findings, especially if the calculated LMP date and ultrasound measurements disagree by more than a week.

Trimester Breakdown

Pregnancy is divided into three trimesters, each roughly 13 weeks long:

• First trimester (weeks 1–13): The period of most rapid development. Major organs form, the heart begins beating (around week 6), and the embryo becomes a fetus by week 10. Miscarriage risk is highest in this trimester.
• Second trimester (weeks 14–27): Often called the "honeymoon trimester." Morning sickness typically eases, energy returns, and you'll likely feel the baby's first movements (quickening) around weeks 16–20. The anatomy scan (usually around week 20) checks fetal development in detail.
• Third trimester (weeks 28–40): The baby gains most of its birth weight during these weeks. Viability outside the womb improves dramatically through this period. Most providers monitor more closely from 36 weeks onward.

Important Milestones in Pregnancy

Beyond trimesters, several specific weeks carry clinical significance:

• Week 8–10: First prenatal appointment for most providers. Heartbeat typically confirmed by ultrasound.
• Week 11–13: First-trimester screening (nuchal translucency ultrasound + blood tests) for chromosomal conditions.
• Week 18–22: Anatomy scan ultrasound. Sex of the baby can typically be determined.
• Week 24: Viability threshold — fetuses born at this point have a survival chance with intensive NICU care.
• Week 28: Third-trimester begins; Rh factor injection if needed; increased monitoring begins for some conditions.
• Week 36–37: Baby considered "late preterm" or "early term." GBS test performed.
• Week 39–40: Full term. Most providers won't induce before 39 weeks without medical indication.
• Week 41–42: Post-dates. Induction is typically discussed or recommended after 41 weeks.

Frequently Asked Questions

• What if I don't know my last period date?

  If you don't remember your LMP, try the conception date method (if you were tracking ovulation) or the ultrasound method. If you have no ultrasound yet, your first prenatal appointment will include one — the provider will assign a gestational age and EDD from that scan.

• My cycle isn't 28 days — how does that affect my due date?

  Our calculator adjusts for your cycle length. For every day your cycle is longer than 28 days, ovulation is approximately one day later, shifting your EDD one day later. For example, a 35-day cycle shifts your due date roughly 7 days later than the standard 28-day calculation.

• Can my due date change?

  Yes. Your provider may revise your EDD based on early ultrasound measurements, especially if the ultrasound-based date differs from the LMP-based date by more than 7 days (in the first trimester) or 10–14 days (in the second trimester). ACOG recommends using ultrasound dating when such discrepancies exist.

• What does "gestational age" mean?

  Gestational age is counted from the first day of your last menstrual period, not from conception. A pregnancy described as "8 weeks gestational age" is approximately 6 weeks post-conception. This is the standard measurement used by OB-GYNs worldwide.

• Is it normal to go past my due date?

  Yes, entirely normal. Only about 5% of babies arrive on their due date. Going a few days past your EDD is very common. Most providers will discuss induction options starting around 41 weeks, and many will recommend induction by 41–42 weeks to avoid the small but real risks associated with post-dates pregnancy.