How Is a Due Date Calculated?
Since the 1800s, Naegele's Rule has been the clinical standard for estimating the Estimated Date of Delivery (EDD). It adds 280 days (40 weeks) to the first day of the Last Menstrual Period (LMP), assuming a 28-day menstrual cycle with ovulation occurring on day 14.
However, biological variability means that cycle lengths and ovulation timing differ between individuals. Our calculator adjusts Naegele's Rule based on your specific cycle length to provide a more personalized and accurate delivery estimate.
Calculation Methods
This calculator supports five clinical methods for determining your due date:
| Method | Formula | Best For |
|---|---|---|
| LMP (Last Menstrual Period) | LMP + 280 days ± cycle adjustment | Most women with regular cycles |
| Conception Date | Conception + 266 days | Women who know exact ovulation date |
| IVF 3-Day Transfer | Transfer + 263 days | IVF patients with Day 3 embryos |
| IVF 5-Day Transfer | Transfer + 261 days | IVF patients with blastocysts |
| Ultrasound | Adjusted based on CRL/gestational age | First-trimester dating scans |
How Accurate Is the Due Date?
Only about 4% of babies are born on their exact estimated due date. Most deliveries occur within a two-week window around the EDD. Accuracy varies by the method and timing of estimation:
First Trimester
Ultrasound dating is highly accurate, within 3-5 days. This is when the definitive EDD is usually established.
Second Trimester
Accuracy drops to 7-10 days as individual genetic growth factors begin to influence fetal size.
Third Trimester
Margin of error increases to 2-3 weeks. Dating is rarely changed this late in the pregnancy.
Key Pregnancy Milestones
- Weeks 1-12 (First Trimester): Major organs form. Morning sickness is common. First prenatal visit and initial ultrasound typically occur.
- Weeks 16-20 (Quickening): You may feel the baby move for the first time. The anatomy scan ultrasound checks structural development.
- Week 24 (Viability): The baby reaches a significant milestone — survival outside the womb becomes possible with intensive medical care.
- Week 37 (Full Term): The pregnancy is considered full term. The baby's lungs and organs are mature enough for independent life.
- Week 40 (Due Date): The estimated date of delivery. Labor can naturally begin anywhere from 38-42 weeks.
When to Call Your Doctor
While due dates are estimates, certain signs warrant immediate medical attention: regular contractions before 37 weeks, vaginal bleeding, sudden decrease in fetal movement, or severe headaches with visual changes. Always follow your healthcare provider's specific guidance for your pregnancy.
Frequently Asked Questions
How is a due date calculated?▼
How accurate is a due date?▼
Can my due date change?▼
How is an IVF due date different?▼
What does 'full term' mean?▼
Disclaimer: This calculator provides estimates based on standard clinical formulas. Your actual delivery date may differ. Always consult your healthcare provider for personalized prenatal care.