How Is Your Due Date Actually Calculated?
The due date you get at your first prenatal visit follows a formula called **Naegele's Rule**, developed by a German obstetrician in the early 1800s. The rule adds **280 days (40 weeks)** from the first day of your last menstrual period. Despite being nearly 200 years old, it remains the standard clinical method used today.
What Methods Can You Use to Estimate Your Due Date?
Depending on your situation, there are three ways to estimate your **pregnancy due date**: from your last period, from your conception date, or from an IVF transfer date. Each uses a different offset to arrive at the same 40-week target, and a good **due date calculator** should support all three.
| Method | Starting Point | Calculation | Best For |
|---|---|---|---|
| Last Menstrual Period (LMP) | First day of last period | Add 280 days | Regular 28-day cycles |
| Conception Date | Ovulation date | Add 266 days | Women tracking ovulation |
| IVF Transfer (Day 5) | Embryo transfer date | Add 261 days | IVF pregnancies |
How Accurate Is the Estimated Due Date?
Only about **5% of babies** arrive on their exact estimated due date. Full-term delivery is considered anywhere between **37 and 42 weeks** — a five-week window. First-trimester ultrasound, measuring crown-rump length at 8–13 weeks, is the most accurate dating method with an error margin of just **±5–7 days**.
Your provider may revise your due date if the ultrasound differs significantly from the LMP-based estimate.
What Are the Key Trimester Milestones?
- **First trimester (Weeks 1–13):** Major organ formation happens. Fatigue, nausea, and breast tenderness are common. Miscarriage risk is highest during this period, which is why many wait until week 12 to share the news.
- **Second trimester (Weeks 14–27):** Energy often returns and morning sickness usually subsides. you'll start feeling fetal movement (quickening) around weeks 16–22. Most prenatal screening tests happen in this window.
- **Third trimester (Weeks 28–40):** Rapid fetal weight gain and lung maturation accelerate. Physical discomfort increases as the baby grows. Braxton Hicks contractions may begin, and you'll start preparing for delivery.
Calculate Your Pregnancy Due Date
Use our Due Date Calculator to get your estimated due date, current gestational age, and trimester schedule. Supports LMP, conception, and IVF dates with automatic cycle-length adjustment.