How is my due date calculated?

Dr. Gabriella C. Pridjian, MD
Maternal & Fetal Medicine Specialist

Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28 day cycle with ovulation at about day 14. If cycles are longer, then adjustments may need to be made. For example if cycles are always 34 days apart, then add an additional 6 days to calculate the due date. Thus, you are actually add 286 days to the first day of your last menstrual period in this case.

Dr. Arlene J. Jacobs, MD
OBGYN (Obstetrician & Gynecologist)

Your due date is calculated based on your last menstrual period. In this video, Arlene Jacobs, MD, an OBGYN with Medical City Frisco in Frisco, Texas, explains this process. 

Boston Women's Health Book Collective
Administration Specialist

A full-term pregnancy lasts approximately 40 weeks, or three trimesters of 13 to 14 weeks each. Because not all pregnancies are equal in duration, it is impossible to predict the precise day on which your baby will be born. Pregnancies usually range from 37 weeks to 43 weeks, with most women giving birth between 39 weeks and 41 weeks after conception. Your due date is merely the middle of this fourteen-day window.

There are several ways to calculate your due date. If you know the exact date of conception, your due date will be 38 weeks from that date. If you know the first day of your last menstrual period, and you have regular cycles of twenty-eight to thirty days, your due date will be 40 weeks from that day.

If you have irregular cycles and do not know the date of conception, or if you cannot remember when your last period started, your provider will suggest getting an early ultrasound, which will measure the size of your baby. The due date determined by an early ultrasound is as accurate as a due date determined by counting days from when your last period started.

Dr. Deborah Raines, MSN
Nursing Specialist

There is no fool proof way to calculate the expected date of birth or due date of a pregnancy. The traditional way to estimate the baby’s due date is Naegles rule. Using Naegles rule, for a woman with a regular 28 day cycles, the expected date of birth (EDB) can be calculated by adding one year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period (LMP). The result is approximately 280 days or 40 weeks.

Since not all women have a regular 28 day cycle, Parikh's Formula is a calculation method that considers cycle duration. Using this formula, the expected date of birth is calculated by adding 9 months to LMP, subtracting 21 days and then adding the duration of previous cycles

However, since the introduction of ultrasound examinations, they are frequently used to date the pregnancy and estimate the expected date of birth.

Continue Learning about Conception Achieved (Pregnancy)

Conception Achieved (Pregnancy)

The process of conception starts in the female reproductive organs. An egg is released into the fallopian tubes once a month to await fertilization. If a sperm reaches the egg and penetrates it, it is fertilized. Conception takes ...

place during ovulation. This happens 2 weeks after the last day of your last menstrual cycle. If eggs are not fertilized, they are released through menstruation and process repeats next month.

Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.