Pregnancy Due Date Calculator & Weekly Guide (280-Day Method)

How to Calculate Pregnancy Weeks

The gestational age (pregnancy weeks) is calculated from the first day of your last menstrual period (LMP). This clinical method of counting means that the first two weeks of your “pregnancy” are actually from before conception occurred. The expected due date is approximately 40 weeks from the LMP. Most people discover their pregnancy around week 5 or 6 of gestational age, when they miss their period. If planning for pregnancy, it’s recommended to use menstrual tracking apps or digital health tools to record your cycles. At your first prenatal visit, your healthcare provider will establish your due date based on your LMP and may adjust it according to early ultrasound measurements.

Due Date Calculation Method

The pregnancy due date is typically calculated using Naegele’s rule, developed by German obstetrician Franz Karl Naegele.

The calculation method uses the first day of the Last Menstrual Period (LMP) as the baseline, then adds one year, subtracts three months, and adds seven days to determine the Estimated Due Date (EDD).

For example, if the first day of the last period was June 1st, “subtract three months, add seven days, add one year” would result in March 8th of the following year.

Generally, the pregnancy term is about 40 weeks, so adding 280 days to the first day of the last menstrual period will yield the same result as Naegele’s rule.

Note: This method assumes a 28-day menstrual cycle with ovulation and fertilization occurring on day 14.

Pregnancy Precautions

Pregnancy can be divided into three trimesters: First trimester (under 13 weeks), Second trimester (13-29 weeks), and Third trimester (29 weeks and above).

• Regular prenatal check-ups
• Wear comfortable, breathable maternity clothes
• Maintain a positive mood
• Avoid secondhand smoke exposure

Regular Prenatal Care

Regular prenatal care includes comprehensive health monitoring for both the pregnant person and developing fetus. These visits include:

• Physical examinations
• Routine blood and urine tests
• Ultrasound screenings
• Genetic screening (if applicable)
• Blood pressure monitoring
• Fetal heart rate monitoring
• Weight tracking
• Nutritional counseling

According to the American College of Obstetricians and Gynecologists (ACOG) and the Office on Women’s Health (OWH), the recommended frequency for routine prenatal visits is:

• Once a month from week 4 through week 28
• Twice a month from week 28 through week 36
• Weekly from week 36 until delivery

Note: High-risk pregnancies may require more frequent prenatal care.

High-Risk Pregnancy

A “high-risk pregnancy” doesn’t necessarily mean problems will occur, but rather indicates a higher probability of complications. According to the Office on Women’s Health (OWH), the following factors may increase the risk of pregnancy complications:

• Being very young or over 35 years old
• Being overweight or underweight
• Having had problems in previous pregnancies
• Pre-existing health conditions such as high blood pressure, diabetes, autoimmune diseases, cancer, and HIV
• Carrying twins or multiples

Health issues that may lead to high-risk pregnancy can also develop during pregnancy, such as gestational diabetes or preeclampsia.

If you have any concerns about high-risk pregnancy, consult your doctor who can explain the risk levels and the actual probability of complications occurring.