Pregnancy Due Date Calculator

How Is a Pregnancy Due Date Calculated?

A pregnancy due date, also called the estimated date of delivery (EDD), is the approximate date when a baby is expected to be born. While only about 5% of babies arrive exactly on their due date, the estimate is a critical reference point for prenatal care, scheduling tests, and monitoring fetal development throughout the pregnancy.

Naegele's Rule

The most widely used method for calculating a due date is Naegele's rule, developed by German obstetrician Franz Naegele in the early 19th century. The formula is straightforward: take the first day of the last menstrual period (LMP), add 280 days (40 weeks), and the result is the estimated due date. This assumes a 28-day menstrual cycle with ovulation occurring around day 14. For women with shorter or longer cycles, the estimate may be adjusted by a healthcare provider.

When the conception date is known instead, the calculation uses 266 days (38 weeks) from the date of conception, since conception typically occurs about two weeks after the start of the last period.

Understanding Gestational Age

Gestational age is measured from the first day of the last menstrual period, not from the date of conception. This means that during the first two weeks of "pregnancy," conception has not yet occurred. Gestational age is expressed in weeks and days (for example, 28 weeks and 3 days) and is the standard way that healthcare providers track pregnancy progress.

Trimester Milestones

A pregnancy is divided into three trimesters, each with distinct developmental milestones:

• First Trimester (Weeks 1–12): All major organs and body systems begin to form. By the end of this trimester, the risk of miscarriage drops substantially. Early prenatal visits, blood work, and the first ultrasound typically occur during this period.
• Second Trimester (Weeks 13–27): Often called the most comfortable trimester, this period includes the anatomy scan around week 20, where detailed images of the baby's organs are reviewed. The baby begins to move noticeably, and the viability milestone is reached around week 24, meaning the baby could potentially survive outside the womb with intensive medical support.
• Third Trimester (Weeks 28–40): The baby gains weight rapidly and the lungs mature. At 37 weeks the pregnancy is considered early term. Regular monitoring increases in frequency, and preparations for delivery begin.

Key Pregnancy Milestones

• 12 Weeks: End of the first trimester. Major organs are formed, and the risk of pregnancy loss decreases significantly.
• 20 Weeks: The mid-pregnancy anatomy scan provides a detailed look at the baby's development and can often reveal the sex of the baby.
• 24 Weeks: The threshold of viability. Babies born at this stage have a chance of survival with advanced neonatal intensive care.
• 37 Weeks: The pregnancy is classified as early term. Babies born at this point generally do well without intensive medical intervention.
• 40 Weeks: The standard estimated due date. Pregnancies that continue past 41 or 42 weeks are considered post-term and may require induction.

Factors That Affect Due Date Accuracy

Due date calculations based on the LMP assume a regular 28-day cycle. Irregular cycles, uncertain period dates, or assisted reproduction can affect accuracy. First-trimester ultrasound measurements, particularly crown-rump length, are often used to confirm or adjust the due date and are generally considered more accurate than LMP-based calculations alone.

Disclaimer

This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Every pregnancy is unique, and only a qualified healthcare provider can give you an accurate due date based on your individual medical history, physical examination, and ultrasound findings. Always consult your doctor, midwife, or obstetrician for personalized prenatal care and guidance.