Pregnancy Due Date Calculator

Calculating your estimated due date (EDD) is one of the first steps in preparing for your pregnancy journey. While the due date is just an estimate – only about 5% of babies are born exactly on their due date – it provides an important reference point for tracking your baby's development and planning.

Calculation Methods

There are several methods used to calculate a due date, each with varying degrees of accuracy depending on your circumstances:

Naegele's Rule (Last Menstrual Period Method)

The most common method is Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method works best for women with regular 28-day cycles and assumes ovulation occurs on day 14. The formula is:

For women with longer or shorter cycles, we adjust the calculation by adding or subtracting days based on the difference from a 28-day cycle.

Conception Date Method

If you know the date of conception, the due date can be calculated by adding 266 days (38 weeks) to that date:

This method can be more accurate if you know when conception occurred, perhaps through tracking ovulation or fertility treatments.

Ultrasound Dating

Especially in the first trimester, ultrasound measurements can provide a more accurate estimation of gestational age. The formula adjusts based on when the ultrasound is performed:

The earlier the ultrasound is performed, the more accurate it is for dating purposes.

IVF Transfer Date

For pregnancies resulting from in vitro fertilization (IVF), the due date can be calculated very precisely:

For example, if a 5-day blastocyst was transferred, the due date would be 261 days after the transfer date.

Pregnancy Timeline

Pregnancy is typically divided into three trimesters, each lasting about 13-14 weeks:

First Trimester (Weeks 1-13)

• Weeks 1-2: These weeks actually occur before conception, counted from the first day of your last period.
• Week 3: Conception typically occurs.
• Weeks 4-5: Implantation happens and pregnancy tests may show positive results.
• Weeks 6-8: The baby's heart begins to beat and basic facial features form.
• Weeks 9-13: All major organs are forming and the baby starts to look more human-like.

Second Trimester (Weeks 14-27)

• Weeks 14-17: Baby's sex becomes visible via ultrasound and movement may be felt.
• Weeks 18-22: Detailed anatomy scan usually performed; baby can hear sounds.
• Weeks 23-27: Baby's lungs develop and viability outside the womb becomes possible.

Third Trimester (Weeks 28-40+)

• Weeks 28-31: Baby gains weight rapidly and can open eyes.
• Weeks 32-35: Lung development continues and baby positions for delivery.
• Weeks 36-40: Full term; baby is ready for birth.
• Weeks 40+: Post-term pregnancy.

Important Terms to Know

Term	Definition
Gestational Age	The time elapsed since the first day of the last menstrual period; the clinical standard for measuring pregnancy duration.
Embryonic Age	The actual time since conception; typically 2 weeks less than gestational age.
Full Term	Pregnancy between 39 weeks 0 days and 40 weeks 6 days.
Early Term	Pregnancy between 37 weeks 0 days and 38 weeks 6 days.
Late Term	Pregnancy between 41 weeks 0 days and 41 weeks 6 days.
Post Term	Pregnancy at 42 weeks 0 days and beyond.

Factors Affecting Due Date Accuracy

Several factors can affect the accuracy of due date predictions:

• Cycle Irregularity: Women with irregular cycles may have less accurate due dates when calculated using LMP method.
• Late Ovulation: Some women ovulate later than day 14, which can lead to a later due date than expected.
• Maternal Factors: Maternal age, prior pregnancies, and health conditions can influence pregnancy duration.
• Multiple Pregnancies: Twins and other multiples typically deliver earlier than singletons.

Note: A due date is an estimate, not a deadline. Only about 5% of babies are born on their actual due date, and delivery within two weeks before or after the due date is considered normal. Always consult with your healthcare provider for personalized guidance throughout your pregnancy.