Pregnancy Due Date Calculator
Estimate your due date based on your last menstrual period, conception date, or IVF transfer date. See trimester timelines, gestational age, and key pregnancy milestones.
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Frequently Asked Questions
How is a pregnancy due date calculated?
The most common method uses Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. For conception dates, 266 days are added. For IVF transfers, 263 days (3-day embryo) or 261 days (5-day embryo) are added to the transfer date.
How accurate is an estimated due date?
Only about 4-5% of babies are born on their exact due date. Most babies are born within a two-week window around the due date. Factors like cycle length, first vs. subsequent pregnancies, and individual variation all affect timing. An ultrasound in the first trimester can refine the estimate.
What are the three trimesters of pregnancy?
The first trimester spans weeks 1-12, during which major organs begin forming. The second trimester covers weeks 13-27, when the baby grows rapidly and movements become noticeable. The third trimester runs from week 28 to delivery (around week 40), as the baby gains weight and prepares for birth.
What is the difference between gestational age and fetal age?
Gestational age is measured from the first day of the last menstrual period, which is about two weeks before conception actually occurs. Fetal age (also called embryonic age) is measured from the date of conception. So gestational age is typically about two weeks longer than fetal age.
Understanding Pregnancy Due Dates
Pregnancy due dates are typically estimated using Naegele's rule, a method developed by German obstetrician Franz Naegele in the early 1800s. The rule assumes a standard 28-day menstrual cycle and calculates the due date as 280 days (40 weeks) from the first day of the last menstrual period. Since ovulation usually occurs around day 14 of the cycle, this effectively estimates 266 days from conception.
While widely used, Naegele's rule has limitations. Women with longer or shorter menstrual cycles may ovulate earlier or later than day 14, shifting the actual due date. First-time mothers tend to deliver slightly later than the estimated date, while subsequent pregnancies may arrive a few days earlier. Ultrasound dating, particularly in the first trimester, provides a more accurate estimate by measuring the embryo's crown-rump length.
A full-term pregnancy ranges from 37 to 42 weeks. Babies born before 37 weeks are considered preterm, while those born after 42 weeks are post-term. Medical professionals use the estimated due date as a reference point for scheduling prenatal tests, monitoring fetal development, and planning delivery. This calculator is intended for informational purposes and should not replace professional medical advice.