Due Date Calculator
Enter the first day of your last menstrual period to calculate your estimated due date using Naegele's rule. Also see your current pregnancy week, days remaining, and trimester.
About the Due Date (Naegele's Rule)
Naegele's rule, developed by German obstetrician Franz Naegele in the 19th century, is the standard method for estimating due dates. It calculates the estimated delivery date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). Only about 5% of babies arrive on the exact due date, but approximately 80% are born within the 38-42 week window.
Method 1: Last Menstrual Period + 280 days (40 weeks)
Method 2: LMP month - 3 months, day + 7, year + 1
Example: LMP January 10 → Due Date: October 17
Fetal Development by Trimester
| Weeks | Period | Baby Size | Key Developments |
|---|---|---|---|
| 1-4 | Early | 0.04 in | Implantation, gestational sac forms |
| 5-8 | Early | 0.6 in | Heartbeat starts, limbs form |
| 9-12 | Early | 2.4 in / 0.6 oz | Major organs complete, fingers & toes develop |
| 13-16 | Mid | 4.7 in / 3.9 oz | Gender identifiable, first movements |
| 17-20 | Mid | 10 in / 10.6 oz | Active movement, hearing develops |
| 21-24 | Mid | 12 in / 1.3 lb | Lung development begins, eyes open |
| 25-28 | Late | 14 in / 2.2 lb | Brain develops rapidly, viability milestone |
| 29-32 | Late | 16 in / 4 lb | Lungs maturing, rapid weight gain |
| 33-36 | Late | 18 in / 5.5 lb | Fat layer builds, immune system develops |
| 37-40 | Full Term | 20 in / 7.3 lb | Ready for birth, head-down position |
Recommended Prenatal Test Schedule (US)
| Timing | Test | Purpose |
|---|---|---|
| 6-8 weeks | Initial ultrasound | Confirm pregnancy, detect heartbeat |
| 11-13 weeks | NT scan + blood work | Screen for chromosomal abnormalities |
| 15-20 weeks | Quad screen / NIPT | Down syndrome and other conditions screening |
| 18-22 weeks | Anatomy scan | Detailed fetal structural examination |
| 24-28 weeks | Glucose screening | Gestational diabetes screening |
| 35-37 weeks | GBS test, NST | Group B strep screening, fetal monitoring |
Cycle Length Adjustment Table
| Cycle Length | Adjustment | Adjusted Due Date (LMP Mar 1) |
|---|---|---|
| 24 days | -4 days | Dec 2 |
| 26 days | -2 days | Dec 4 |
| 28 days (standard) | 0 days | Dec 6 |
| 30 days | +2 days | Dec 8 |
| 32 days | +4 days | Dec 10 |
| 35 days | +7 days | Dec 13 |
Delivery Preparation Checklist
- Week 20: Begin childbirth education classes, start a birth plan
- Week 28: Start packing your hospital bag, discuss birth preferences with provider
- Week 32: Prepare nursery and baby essentials (car seat, crib, stroller)
- Week 34: Finalize hospital bag, pre-register at your hospital or birth center
- Week 36: Install and inspect infant car seat, confirm route to hospital
- Week 37+: Full term -- be ready for labor at any time
Due Date Accuracy & Statistics
According to research published in the American Journal of Obstetrics and Gynecology, only about 4-5% of babies are born on the exact estimated due date. About 50% deliver within 1 week of the due date, and 80% within 2 weeks (38-42 weeks). The average US pregnancy lasts about 39 weeks and 2 days. The US cesarean delivery rate is approximately 32%, and the preterm birth rate is about 10.4% (CDC data). First-time mothers average 40 weeks and 5 days, while experienced mothers deliver closer to 40 weeks.
Important Notes
- Naegele's rule assumes a regular 28-day cycle. If your cycle is different, use the adjustment table above.
- The estimated due date is an approximation -- always confirm with your healthcare provider.
- Seek immediate medical attention for heavy bleeding, severe abdominal pain, or fluid leaking.
- Do not take any medications during pregnancy without consulting your doctor.
- Advanced maternal age (35+) may warrant additional screening tests such as NIPT or amniocentesis.
- Report any signs of gestational diabetes or preeclampsia (high blood pressure, swelling, vision changes) to your provider immediately.