How to Calculate Pregnancy Due Date
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Understanding Your Due Date
Your due date is an estimate of when your baby will be born. Think of it as your baby's "expected arrival date" rather than an exact deadline!
Important to know: Only about 4% of babies are actually born on their due date. Most babies arrive within 2 weeks before or after this date, which is completely normal.
How We Calculate Due Dates
📅 Last Menstrual Period (LMP)
We count 280 days (40 weeks) from the first day of your last period. This is the most common method doctors use.
🔊 Ultrasound Dating
Uses sound waves to measure your baby and compare growth to typical development patterns. Most accurate in early pregnancy.
🎯 Conception Date
If you know when you conceived, we add about 38 weeks (266 days) to that date.
🧪 IVF Transfer
The most precise method since we know the exact fertilization date. We calculate based on embryo age and transfer date.
When Will Your Baby Actually Arrive?
37-39 weeks: Early Term
Baby is developed enough to be born safely, but full-term is still better.
39-41 weeks: Full Term
The ideal time for birth. Baby has had time to fully develop.
41-42 weeks: Late Term
Still normal, but your doctor will monitor you more closely.
Remember: Your due date is a helpful guide, not a strict deadline. Your baby will come when they're ready! Use this time to prepare and stay in touch with your healthcare provider.
Manual Method: Naegele's Rule
Want to calculate by hand? Here's the traditional method doctors have used for over 150 years:
- Take the first day of your last menstrual period
- Add 280 days (40 weeks)
- Or use the shortcut: subtract 3 months, then add 7 days
Example: Last period started January 1st → Due date is October 8th (January 1st - 3 months + 7 days)
Frequently Asked Questions
What if I have irregular periods or missed periods?
If you have irregular cycles, the LMP method may be less accurate. In this case, an ultrasound dating scan (usually done between 8-14 weeks) provides the most accurate due date. Consult with your healthcare provider for personalized guidance.
How does cycle length affect the calculation?
Naegele's Rule assumes a 28-day cycle. If your cycle is longer or shorter, your due date may vary. For cycles longer than 28 days, add the extra days to your due date. For shorter cycles, subtract the difference.
How accurate are due date calculations?
Due date calculations are estimates. Only about 5% of babies are born on their exact due date. Most babies (about 90%) are born within two weeks before or after the due date. This is considered "full-term."
When should I know my conception date?
You might know your conception date if you were tracking ovulation, used fertility treatments, or have a very regular cycle. Conception typically occurs around 14 days after the start of your last period in a 28-day cycle.
What's the difference between gestational age and fetal age?
Gestational age is calculated from your LMP and is about 2 weeks longer than fetal age. Fetal age (or embryonic age) is calculated from conception. Medical professionals typically use gestational age for consistency.
How accurate are ultrasound due date calculations?
Ultrasound dating is most accurate when performed in the first trimester (before 14 weeks). Early ultrasounds can predict due dates within 3-5 days. Later ultrasounds become less accurate as fetal growth varies more between pregnancies.
How do IVF due dates differ from natural conception?
IVF due dates are often more precise because the exact fertilization date is known. For Day 3 embryo transfers, subtract 17 days from transfer date to get LMP equivalent. For Day 5 blastocysts, subtract 19 days. This makes IVF dating very accurate.
Which calculation method should I use?
Use the most accurate information you have: IVF transfer dates are most precise, followed by early ultrasound measurements, then conception date if known, and finally LMP for regular cycles. Your healthcare provider may adjust based on ultrasound findings.