Pregnancy Due Date: Calculate From Last Period

The Formula Doctors Have Used for 200 Years

The standard way to calculate a pregnancy due date goes back to a German obstetrician named Franz Naegele, who came up with it in the early 1800s. It’s called Naegele’s Rule, and it’s still the foundation of modern due date estimation.

Here’s the formula: take the first day of your last menstrual period (LMP), add one year, subtract three months, and add seven days.

In practice, most people just count 280 days (40 weeks) from their LMP. Same result, less math.

Quick Example

Say your last period started on January 1:

1. Add one year: January 1 of the following year
2. Subtract three months: October 1
3. Add seven days: October 8

Your estimated due date? October 8.

Wait — Why Your Last Period and Not Conception?

This confuses almost everyone. Pregnancy is dated from the first day of your last period, not from when you actually conceived. Since most women ovulate about 14 days after their period starts, you’re technically counted as “two weeks pregnant” on the day of conception. Weird, right?

The convention exists because most women know exactly when their last period started, but pinpointing the actual conception date is tricky. Sperm can survive in the reproductive tract for up to five days, so conception might not even happen on the day you had sex.

So a “40-week pregnancy” really includes about 38 weeks of actual fetal development — plus those 2 weeks before conception even happened.

How Accurate Is Your Due Date, Really?

Only about 4 to 5% of babies show up on their exact due date. Four to five percent. But the due date still gives you a useful target. Most births fall within a two-week window around it:

• 37-38 weeks: Early term (healthy but a bit ahead of schedule)
• 39-40 weeks: Full term (the sweet spot)
• 41 weeks: Late term
• 42+ weeks: Post-term (most providers will talk induction at this point)

First babies tend to take their time. The average first-time mom delivers at 40 weeks and 5 days. Later pregnancies usually come a touch earlier.

When Your Due Date Gets Changed

Your provider might adjust your due date based on a first-trimester ultrasound. Measurements taken between 8 and 13 weeks are accurate to within 5 to 7 days. If the ultrasound date and your LMP-based date differ by more than 7 days, most practices will go with the ultrasound.

Why is first-trimester dating more accurate? Because not all women ovulate on day 14, and not all cycles are 28 days long. The ultrasound cuts through those variables.

Your Three Trimesters — What to Expect

Once you’ve got your due date, you can map out what’s ahead:

First Trimester (Weeks 1-12)

This is when the most dramatic development happens. By week 12, all major organs have formed, the heart is beating, and the fetus is about 2.5 inches long. But it’s also when you might feel the roughest:

• Nausea and vomiting (peaks around weeks 8-10 — hang in there)
• Crushing fatigue
• Breast tenderness
• Running to the bathroom constantly
• Wild food aversions or cravings

The first trimester carries the highest miscarriage risk, but that risk drops significantly once a heartbeat is confirmed around week 8, and drops further after week 12.

Second Trimester (Weeks 13-27)

Often called the “golden trimester” — and for good reason. Nausea usually fades, energy bounces back, and your bump starts showing. Major milestones include:

• Feeling baby’s first movements (quickening) around weeks 18-22
• The anatomy scan ultrasound at weeks 18-20
• Finding out the sex — if you want to — at the anatomy scan
• Glucose screening for gestational diabetes at weeks 24-28

Third Trimester (Weeks 28-40)

The home stretch. Baby is packing on weight fast — about half a pound per week during weeks 28 to 36. Meanwhile, you’re dealing with:

• Braxton Hicks contractions (practice rounds)
• Shortness of breath as your uterus pushes up against your diaphragm
• Back pain and pelvic pressure
• Sleep becoming… creative
• More frequent prenatal visits (weekly after 36 weeks)

What If Your Cycles Aren’t Regular?

Naegele’s Rule assumes a 28-day cycle with ovulation on day 14. If that doesn’t describe you, here’s how to adjust:

• 35-day cycle: Add 287 days to your LMP instead of 280 (because you likely ovulate around day 21 instead of day 14)
• 21-day cycle: Add 273 days to your LMP

If your cycles are really unpredictable, a first-trimester ultrasound is your most reliable option. Talk to your provider about early dating if your cycle length bounces around by more than a week.

Your First Trimester Appointment Roadmap

1. First visit (weeks 6-8): Confirm the pregnancy, estimate due date, order bloodwork, talk prenatal vitamins
2. First ultrasound (weeks 8-10): Confirm viability, check for heartbeat, date the pregnancy
3. Genetic screening (weeks 10-13): NIPT blood test or nuchal translucency ultrasound to screen for chromosomal conditions
4. End of first trimester visit (week 12): Review results, map out second trimester plans

If you’re still in the planning stage, our Ovulation Calculator can help you zero in on your most fertile days.