Understanding Pregnancy Dating Methods
Our pregnancy calculator provides accurate due date estimation using four different calculation methods, each suitable for different pregnancy scenarios:
Last Menstrual Period (LMP) Method
The most common method used by healthcare providers. Pregnancy typically lasts 40 weeks (280 days) from the first day of your last menstrual period. This method assumes a 28-day cycle with ovulation occurring on day 14.
Conception Date Method
For when you know your exact conception date (like with fertility tracking). Pregnancy is calculated as 38 weeks (266 days) from conception. This method is particularly accurate if you know your ovulation date.
Ultrasound Dating
The most accurate method, especially when performed in the first trimester. Ultrasound measurements of the fetus are compared to standard growth charts to estimate gestational age and due date.
IVF Transfer Dating
Specialized calculation for IVF pregnancies. The due date is calculated based on the embryo transfer date and the age of the embryo at transfer (3-day or 5-day embryo).
Pregnancy Timeline Facts
- • Only about 5% of women deliver on their exact calculated due date
- • A normal pregnancy can last between 37-42 weeks (full term)
- • First trimester: Weeks 1-13 (highest risk of miscarriage)
- • Second trimester: Weeks 14-27 (often most comfortable period)
- • Third trimester: Weeks 28-40+ (final growth and preparation for birth)
- • Age of viability (when baby could survive outside womb): About 24 weeks
Important Considerations
Cycle Variations: Women with irregular cycles may need ultrasound dating for accuracy. Our calculator allows custom cycle length adjustments.
IVF Pregnancies: The exact embryo age at transfer is crucial for accurate dating in IVF conceptions.
Medical Advice: Always consult with your obstetrician or midwife about your specific pregnancy timeline and due date.
Disclaimer: This calculator provides estimates only and should not replace professional medical advice.
Frequently Asked Questions
Q: Why is my due date different from what my doctor calculated?
A: Doctors may adjust dates based on ultrasound measurements or other clinical factors. Early ultrasound dating (especially before 13 weeks) is considered most accurate.
Q: How accurate are due date calculators?
A: When using LMP with regular cycles, they're about ±5 days accurate. Ultrasound dating can be accurate to ±3-5 days in first trimester, increasing to ±2-3 weeks later in pregnancy.
Q: Can my due date change during pregnancy?
A: Yes, especially if early ultrasounds suggest different dating than your LMP calculation. Most providers won't change dates after the first trimester.