Due Date Calculator: Your Complete Guide
How is the due date calculated?
The estimated due date (EDD) is calculated using Naegele's Rule, developed by German obstetrician Franz Naegele in the 19th century. This method remains the standard used by healthcare professionals worldwide.
Naegele's Rule Formula
EDD = Last Menstrual Period (LMP) + 280 days (40 weeks)
Formula variables:
- LMP = First day of your last menstrual period
- 280 days = Standard pregnancy duration (40 weeks)
If you know the conception date, the formula is different:
EDD = Conception Date + 266 days (38 weeks)
Cycle Length Adjustment
The standard calculation assumes a 28-day cycle. If your cycle is longer or shorter, the date is adjusted:
- 35-day cycle: Add 7 days to the EDD
- 21-day cycle: Subtract 7 days from the EDD
Adjustment = (Your cycle length - 28) days
The Three Trimesters of Pregnancy
Pregnancy is divided into three trimesters, each with specific characteristics:
| Trimester | Weeks | Main Development | |-----------|-------|-----------------| | First | 1-12 | Organ formation | | Second | 13-26 | Rapid growth | | Third | 27-40 | Final maturation |
Important Dates During Pregnancy
- Week 12: First ultrasound and first trimester screening
- Week 20: Anatomy scan (detailed ultrasound)
- Week 24: Fetal viability milestone
- Week 28: Glucose tolerance test
- Week 37: Beginning of full-term pregnancy
How Accurate is the Estimated Date?
Only approximately 5% of babies are born exactly on their due date. Most babies are born between weeks 37 and 42, which is considered a normal range. This calculator provides an estimate, not an exact prediction.
Frequently Asked Questions
The estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. If you know the conception date, add 266 days (38 weeks) from that date. This is the same formula used by doctors and midwives worldwide.
Only about 5% of babies are born exactly on their estimated due date. Most babies are born between weeks 37 and 42, which is considered a completely normal range. The date is a guide, not an exact prediction, and your doctor may adjust it based on ultrasound measurements.
The standard calculation assumes a 28-day menstrual cycle with ovulation on day 14. If your cycle is longer (for example, 35 days), ovulation occurs later, so the estimated date is adjusted by adding the extra days. The opposite applies for shorter cycles.
The first trimester (weeks 1-12) is when all major organs of the baby form. The second trimester (weeks 13-26) is characterized by rapid growth and is typically when you start feeling the baby move. The third trimester (weeks 27-40) is the final maturation stage where the baby gains weight and the lungs prepare for breathing.
The anatomy or morphology ultrasound is typically performed between weeks 18 and 22 of pregnancy, with week 20 being the most common. This detailed ultrasound evaluates the baby's anatomy, including the brain, heart, spine, and limbs, and is when many parents find out the baby's sex if they choose to.
A pregnancy is considered full-term when it reaches 37 weeks. Babies born between weeks 37 and 42 are considered term births. Before 37 weeks is considered premature. Most healthcare providers prefer to wait until at least 39 weeks for scheduled inductions, as each additional week benefits the baby's development.
Yes, the estimated due date can be adjusted, especially after the first ultrasound. Baby measurements from the first-trimester ultrasound are very accurate for determining gestational age. If there's a significant difference between the date calculated by LMP and the ultrasound, your doctor may adjust the official date.
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