How is the due date calculated?
This calculator uses Naegele's Rule, the standard method used by doctors:
If using a conception date, the calculation adds 266 days (38 weeks) since conception occurs approximately 2 weeks after the LMP.
The due date is an estimate — a normal pregnancy ranges from 38 to 42 weeks. Only about 5% of babies arrive on the exact predicted date.
Trimester overview
| Trimester | Weeks | Key developments |
|---|---|---|
| First | 1–12 | Implantation, organ formation, heartbeat detectable by week 6–7 |
| Second | 13–26 | Baby's movements felt, gender visible on ultrasound, rapid growth |
| Third | 27–40 | Brain development, weight gain, preparation for birth |
Key pregnancy milestones
| Week | Milestone |
|---|---|
| 6–7 | Heartbeat detectable on ultrasound |
| 12 | First trimester screening / NT scan |
| 16–20 | First movements felt (quickening) |
| 18–22 | Anomaly scan (detailed ultrasound) |
| 24 | Viability milestone |
| 28 | Third trimester begins, glucose test |
| 36 | Baby typically moves to head-down position |
| 37 | Considered early term |
| 39–40 | Full term, expected delivery |
Frequently asked questions
What if I don't know my LMP date?
If your periods are irregular or you don't remember the date, an early ultrasound (ideally before 12 weeks) can estimate gestational age very accurately — usually within 3–5 days.
Does the due date change after an ultrasound?
If the ultrasound-estimated age differs from the LMP-based estimate by more than 7 days (in the first trimester), your doctor may adjust the due date to match the ultrasound.
What is the difference between gestational age and fetal age?
Gestational age is counted from the LMP (used for due date calculation and is about 2 weeks ahead). Fetal age is counted from conception. A baby at "40 weeks gestational age" is actually about 38 weeks old from conception.
Is it normal to deliver before or after the due date?
Yes. Only about 5% of babies arrive on the exact due date. About 80% are born between weeks 38 and 42. Delivery before 37 weeks is considered preterm, and after 42 weeks is post-term.