Using the Pregnancy Calculator: Due Date & Milestones
How due date calculation works using Naegele's Rule and LMP (Last Menstrual Period), what each trimester involves, and how to use our pregnancy calculator to track key milestones.
Related Calculator
Use the Pregnancy Calculator to apply what you learn in this guide.
What Is a Pregnancy Due Date?
A pregnancy due date (also called the Estimated Due Date or EDD) is the projected date of delivery, calculated as approximately 280 days (40 weeks) from the first day of the last menstrual period (LMP). It represents the midpoint of a range — most healthy births occur between 37 and 42 weeks of gestation.
Only about 5% of babies are born on their exact due date. The EDD is best understood as the center of a 5-week window (38–42 weeks) during which labor is most likely to begin naturally.
The Formula: Naegele's Rule
The most widely used method for calculating a due date is Naegele's Rule, developed in the 19th century and still used by OBs worldwide:
$$ \text{EDD} = \text{LMP} + 280 \text{ days} $$
Or equivalently, using the shorthand:
$$ \text{EDD} = \text{LMP} + 9 \text{ months} + 7 \text{ days} $$
Example: If your last menstrual period began on September 15, 2025:
$$ \text{EDD} = \text{September 15} + 280 \text{ days} = \textbf{June 22, 2026} $$
Naegele's Rule assumes:
- A regular 28-day menstrual cycle
- Ovulation occurring on Day 14
- Fertilization occurring on Day 14
If your cycle is longer or shorter than 28 days, a corrected formula adjusts the EDD:
$$ \text{EDD (adjusted)} = \text{LMP} + 280 + (\text{cycle length} - 28) \text{ days} $$
A woman with a 35-day cycle would add 7 extra days to the standard EDD.
Step-by-Step Guide
Step 1: Identify Your LMP Start Date
The LMP is the first day of your most recent menstrual period before pregnancy — not the day it ended. This date is the foundation of all gestational age calculations.
Step 2: Add 280 Days (or Use Our Calculator)
Count forward 280 days from your LMP, or simply enter the date into our pregnancy calculator. The calculator also adjusts for non-standard cycle lengths automatically.
Step 3: Confirm with Ultrasound
A first-trimester ultrasound (8–13 weeks) is the most accurate way to confirm or adjust the EDD. Ultrasound dating is more accurate than LMP when:
- Cycle is irregular or unknown
- LMP is uncertain
- LMP-based EDD and ultrasound-based EDD differ by more than 7 days
Step 4: Track Trimester Milestones
Once you have your EDD, work backward to identify which trimester you're in and what key appointments are approaching.
Pregnancy Milestones Table
| Week | Milestone |
|---|---|
| Week 4–5 | Positive home pregnancy test |
| Week 6 | Fetal heartbeat detectable via transvaginal ultrasound |
| Week 8–10 | First prenatal appointment; NIPT (genetic) blood test offered |
| Week 11–13 | First-trimester screening ultrasound (nuchal translucency) |
| Week 16–20 | Quad screen blood test for chromosomal abnormalities |
| Week 20 | Anatomy scan (mid-pregnancy ultrasound) |
| Week 24 | Threshold of viability — advanced NICU care possible |
| Week 28 | Third trimester begins; glucose tolerance test |
| Week 36 | GBS (Group B Strep) swab test |
| Week 39 | Full term — optimal delivery window begins |
| Week 40 | EDD (estimated due date) |
| Week 42 | Post-term; medical induction typically discussed |
Trimester Breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | Weeks 1–13 | Organ formation, heartbeat, risk of miscarriage highest, morning sickness |
| Second | Weeks 14–26 | Movement felt (~week 18–22), anatomy scan, lowest-risk trimester |
| Third | Weeks 27–40 | Rapid weight gain, lung maturation, birth preparation, Group B Strep test |
IVF Pregnancies: A Different Starting Point
For pregnancies achieved through In Vitro Fertilization (IVF), the due date is calculated differently because the exact fertilization date is known:
- Day 3 Transfer: Add 263 days to the transfer date
- Day 5 Transfer (Blastocyst): Add 261 days to the transfer date
IVF due dates are generally more precise than LMP-based calculations.
Key Concepts
| Term | Definition |
|---|---|
| LMP | Last Menstrual Period; Day 1 = start of pregnancy countdown |
| Gestational Age | Age of the pregnancy from LMP (not from conception) |
| Naegele's Rule | EDD = LMP + 280 days; the standard due date formula |
| Trimester | One of three ~13-week stages of pregnancy |
| Viability | ~24 weeks; age at which survival outside the womb becomes possible |
| Full Term | 39–40 weeks; optimal gestational age for delivery |
| EDD | Estimated Due Date; the projected date of delivery |
Frequently Asked Questions
Why is pregnancy counted from the LMP rather than conception? Conception (ovulation and fertilization) typically occurs 2 weeks after the LMP, but the exact date of ovulation is rarely known. Using LMP as Day 1 provides a consistent, documentable reference point that can be confirmed by both the patient and ultrasound measurements.
My ultrasound date differs from my LMP date — which is correct? Before 13 weeks, ultrasound dating is more accurate than LMP because it directly measures fetal size. If the discrepancy is 7 days or more in the first trimester, OBs typically adjust the EDD to the ultrasound-based date.
Can I choose my due date? No, the EDD is a medical calculation based on gestational age. However, planned C-sections and inductions are typically scheduled between 39–42 weeks, giving some predictability.
What does "post-term" mean? A pregnancy is post-term at 42 weeks (294 days from LMP). At this stage, placental function begins declining, increasing risks for the baby. Most OBs recommend discussing induction by 41–42 weeks.