How to Calculate Your Pregnancy Due Date

Your pregnancy due date is one of the first numbers your provider calculates — and one of the most misunderstood. Learning how to calculate your pregnancy due date helps you understand where that number comes from, how precise it really is, and what it means for your prenatal care timeline. The math is simpler than most people expect, and knowing the formula helps you cross-check any estimate and plan accordingly.

How to calculate your pregnancy due date using Naegele's Rule

The standard method for calculating a due date is Naegele's Rule, named after the 19th-century German obstetrician Franz Karl Naegele. It starts from the first day of your last menstrual period (LMP) and works forward:

1. Note the first day of your last menstrual period.
2. Add one year to that date.
3. Subtract 3 months.
4. Add 7 days.

The result is your estimated due date (EDD). You can also express this as: LMP + 280 days, since 280 days equals 40 weeks, and 40 weeks is the conventional length of a full-term pregnancy.

Worked example

Say your last menstrual period started on November 10, 2025.

• Add one year: November 10, 2026
• Subtract 3 months: August 10, 2026
• Add 7 days: August 17, 2026

Your estimated due date is August 17, 2026. Alternatively: November 10, 2025 + 280 days = August 17, 2026. Both routes give the same answer.

Rather than doing the arithmetic by hand, the pregnancy due date calculator handles it instantly — enter your LMP and it returns your due date, current trimester, and exactly how many weeks along you are today.

Why pregnancy is counted from the last period, not conception

This often surprises people: pregnancy is counted starting from the first day of your last menstrual period, even though conception typically happens about two weeks after that date. The reason is practical — most women can pinpoint their LMP date but not their exact ovulation or conception date.

By convention, a "40-week pregnancy" includes roughly two weeks before conception actually occurred. This means at the moment the embryo implants (around week 3 of the pregnancy count), the pregnancy is already being tracked as 3 weeks along — even though fertilization happened only about a week earlier.

If you know your conception date more precisely — for example, if you tracked ovulation or used assisted reproduction — you can calculate the due date differently: conception date + 266 days (38 weeks). The due date calculator supports both methods: LMP and known conception date.

What if my cycle isn't 28 days?

Naegele's Rule assumes a standard 28-day menstrual cycle with ovulation on day 14. If your cycle is consistently longer or shorter than 28 days, the raw LMP calculation will be off.

• Shorter cycles (e.g., 24 days): Ovulation likely happens earlier than day 14, so conception occurred sooner. Your true due date is a few days earlier than Naegele's formula suggests.
• Longer cycles (e.g., 35 days): Ovulation likely happens later, pushing the estimated due date forward by a week or more.
• Irregular cycles: The LMP method may be unreliable. An early ultrasound will give a more accurate estimate.

The adjustment is simple: for each day your average cycle differs from 28 days, shift the due date by that same number of days. A 35-day cycle means a due date about 7 days later than the LMP calculation; a 21-day cycle means about 7 days earlier.

How an ultrasound can revise your due date

An early ultrasound — typically done between 8 and 13 weeks of pregnancy — measures the fetus from crown to rump. This crown-rump length (CRL) correlates closely with gestational age and is the most accurate way to date a pregnancy in the first trimester.

If the ultrasound measurement differs from the LMP-based date by more than 5–7 days, most providers will redate the pregnancy to match the ultrasound. This is especially common when:

• The patient has irregular periods or is uncertain of her LMP date.
• The LMP-based calculation and ultrasound disagree by more than a week.
• The patient conceived through IVF or other assisted reproduction, where the transfer date provides a more precise starting point.

After 13 weeks, ultrasound dating becomes less precise because fetal growth rates vary more between individuals. A due date established from a first-trimester ultrasound is generally considered more reliable than one based on LMP alone.

Pregnancy trimesters explained

Pregnancy is divided into three trimesters. The boundaries are counted from the LMP-based gestational age:

• First trimester: Weeks 1–13 (conception through ~month 3). Major organ development occurs. Most miscarriage risk is concentrated here.
• Second trimester: Weeks 14–27 (months 4–6). Often called the "honeymoon trimester" — nausea typically subsides and energy improves. Fetal movement (quickening) usually begins around weeks 18–22.
• Third trimester: Weeks 28–40+ (months 7–9). Rapid fetal weight gain. The baby moves into birth position. The final weeks involve monitoring for labor signs.

Full term and what due dates actually mean

A common misconception is that the due date is a deadline or a target. In reality, it is the midpoint of an expected delivery range. Medically, pregnancy terms are categorized as:

• Early term: 37–38 weeks, 6 days
• Full term: 39–40 weeks, 6 days
• Late term: 41–41 weeks, 6 days
• Post-term: 42 weeks and beyond

Only about 4–5% of babies are born exactly on their due date. Most arrive within a two-week window — one week before to one week after. If labor hasn't started by 41–42 weeks, most providers discuss induction to avoid complications associated with post-term pregnancy.

Knowing your exact gestational age in weeks and days is also useful for tracking developmental milestones — what size the baby is this week, which prenatal tests are recommended at which intervals, and when to schedule appointments. The pregnancy due date calculator gives you all of that in one place.

Tracking your health during pregnancy

Beyond the due date, pregnancy involves a series of weight, nutrition, and health benchmarks. Understanding where you are in the pregnancy timeline helps you make sense of your provider's recommendations at each visit. Gestational weight gain guidelines, for example, depend on your pre-pregnancy BMI — a metric you can check quickly with the BMI calculator. Knowing your starting point gives you and your provider a clearer picture of appropriate weight gain targets for each trimester.

If you're also wondering how your calorie needs shift during pregnancy, the general guidance from the American College of Obstetricians and Gynecologists is that additional calories are minimal in the first trimester (roughly 0 extra per day), increase to about 340 extra per day in the second trimester, and about 450 extra per day in the third trimester. The calorie needs guide covers how to calculate your baseline TDEE — the starting point before any pregnancy adjustments.

Your estimated due date is a useful planning anchor, not a precise prediction. The LMP method gives a quick, reliable first estimate; an early ultrasound refines it. Whether you're a few weeks in or approaching that final stretch, knowing the math behind the number helps you interpret every appointment, milestone, and week count with a clearer head.