Pregnancy Due Date: Calculate From Last Period

How Due Dates Are Calculated

The standard method for calculating a pregnancy due date is called Naegele’s Rule, named after German obstetrician Franz Naegele. It was developed in the early 1800s and remains the foundation of modern due date estimation.

The formula is simple: 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 add 280 days (40 weeks) to their LMP date. That is mathematically the same thing.

A Quick Example

If 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 would be October 8.

Why the Calculation Uses Your Last Period, Not Conception

This confuses many people. Pregnancy is dated from the first day of your last menstrual period, not from the day you actually conceived. Since most women ovulate about 14 days after their period starts, you are technically “two weeks pregnant” on the day of conception.

This convention exists because the LMP date is usually known with certainty, while the exact date of conception is harder to pinpoint. Sperm can survive in the reproductive tract for up to five days, so conception may not occur on the day of intercourse.

A standard 40-week pregnancy therefore includes approximately 38 weeks of actual fetal development and about 2 weeks before conception even occurred.

How Accurate Are Due Dates?

Only about 4 to 5 percent of babies arrive on their exact due date. However, the due date gives a useful target. Most births occur within a two-week window around the due date:

• 37-38 weeks: Early term (healthy but slightly early)
• 39-40 weeks: Full term (ideal)
• 41 weeks: Late term
• 42+ weeks: Post-term (most providers will recommend induction)

First pregnancies tend to go longer. The average first-time mother delivers at 40 weeks and 5 days. Subsequent pregnancies tend to deliver slightly earlier.

When Due Dates Get Adjusted

Your provider may 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 differs from your LMP-based date by more than 7 days, most practices will change the due date.

First-trimester dating is more accurate than LMP calculation because not all women ovulate on day 14 and not all cycles are 28 days long.

Your Trimester Breakdown

Once you have your due date, you can map out the three trimesters:

First Trimester (Weeks 1-12)

This is the period of the most rapid development. By week 12, all major organs have formed, the heart is beating, and the fetus is about 2.5 inches long. Common symptoms include:

• Nausea and vomiting (peaks around weeks 8-10)
• Extreme fatigue
• Breast tenderness
• Frequent urination
• Food aversions or cravings

The first trimester also carries the highest risk of miscarriage, with the risk dropping significantly after a heartbeat is confirmed at 8 weeks and further declining after 12 weeks.

Second Trimester (Weeks 13-27)

Often called the “golden trimester” because nausea typically subsides, energy returns, and the belly grows visibly. Key milestones include:

• Feeling first fetal movements (quickening) around weeks 18-22
• Anatomy scan ultrasound at weeks 18-20
• Finding out the sex (if desired) at the anatomy scan
• Glucose screening for gestational diabetes at weeks 24-28

Third Trimester (Weeks 28-40)

The final stretch brings rapid fetal weight gain and increasing physical discomfort for the mother. The baby gains about half a pound per week during weeks 28 to 36. Expect:

• Braxton Hicks contractions (practice contractions)
• Shortness of breath as the uterus pushes against the diaphragm
• Back pain and pelvic pressure
• Difficulty sleeping
• More frequent prenatal visits (weekly after 36 weeks)

What If You Have Irregular Cycles?

Naegele’s Rule assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, you can adjust:

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

If your cycles are very irregular, a first-trimester ultrasound is the most reliable way to establish your due date. Talk to your provider about early dating if your cycle length varies by more than a week from month to month.

Key Prenatal Appointments in the First Trimester

1. First visit (weeks 6-8): Confirm pregnancy, estimate due date, order blood work, discuss 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 test results, discuss second trimester plans

If you are still in the planning stage, our Ovulation Calculator can help you identify your most fertile days.