Calculating Your Due Date
Standard pregnancy duration is 40 weeks (280 days) from the first day of the last menstrual period (LMP). Actual delivery typically falls between week 37 and week 42.
Naegele's Rule (Standard Method)
Due Date = LMP + 280 days
= LMP + 9 months + 7 days
Example: LMP = January 1
+9 months = October 1
+7 days = October 8
Due date ≈ October 8Weeks Pregnant
Weeks pregnant = (Today - LMP) / 7
Days pregnant = Today - LMP
If LMP was 10 weeks ago:
Weeks = 10w0d
Gestation = first day of week 11 beginsTrimester Breakdown
- First trimester: Weeks 1-12 (LMP to 12+6) — organ formation
- Second trimester: Weeks 13-26 — rapid growth, movement felt
- Third trimester: Weeks 27-40 — lung maturation, weight gain
- Term: 37-40 weeks (full term), 40-42 weeks (late term)
- Post-term: ≥42 weeks (induction usually recommended)
Calculate due date: Free Pregnancy Calculator
Pregnancy Milestone Quick-Reference Table
| Week | Trimester | Key milestone |
|---|---|---|
| 4 | First | Missed period; positive pregnancy test |
| 6–8 | First | Heartbeat detectable by transvaginal ultrasound |
| 10–13 | First | Nuchal translucency scan; NIPT blood test available |
| 12 | End of First | Miscarriage risk drops significantly |
| 18–22 | Second | Anatomy scan (mid-pregnancy ultrasound) |
| 20 | Second | Halfway point; quickening (fetal movement) felt |
| 24 | Third begins | Viability threshold (intensive care survival possible) |
| 37 | Third | Full term (previously "early term") |
| 40 | Third | Estimated due date (EDD) |
| 42 | Post-term | Induction typically offered |
How Pregnancy Due Date Calculation Works
The standard method is Naegele's Rule: EDD = LMP (first day of last menstrual period) + 280 days (40 weeks). This assumes a 28-day cycle with ovulation on day 14. For irregular cycles, ultrasound dating in the first trimester (crown-rump length measurement) is more accurate than LMP — within ±5–7 days before 13 weeks. After 13 weeks, ultrasound accuracy decreases to ±2–3 weeks.
Pregnancy is divided into three trimesters: first (weeks 1–12), second (weeks 13–28), and third (weeks 29–40). Gestational age is counted from LMP — which means the embryo is actually 2 weeks younger than the gestational age. Full-term pregnancy is now defined as 39–40 weeks; "early term" is 37–38 weeks; preterm is before 37 weeks. Only about 4% of babies are born on their exact EDD; 80% are born within 10 days either side.
Common Mistakes
- Treating the due date as a deadline: EDD is a statistical midpoint, not a definitive prediction. Normal delivery ranges from 37 to 42 weeks. Expecting or planning around the exact date causes unnecessary stress when labour doesn't begin precisely on schedule.
- Using LMP for irregular cycles without adjustment: Naegele's Rule is calibrated for 28-day cycles. With a 35-day cycle, ovulation occurs around day 21, making the EDD approximately 7 days later than the standard calculation. Ultrasound confirmation is important for anyone with irregular cycles.
- Confusing weeks pregnant with months pregnant: Pregnancy lasts about 9 calendar months (10 lunar months). Week 20 is approximately month 4.5, not month 5. The "how many months?" calculation can differ depending on whether you count from conception or LMP.
Frequently Asked Questions
Gestational age (measured from LMP) is the clinical standard used by doctors worldwide. Fetal age (measured from conception/fertilisation) is approximately 2 weeks less than gestational age, since LMP occurs about 2 weeks before ovulation. A pregnancy at 10 weeks gestational age has a fetus that is ~8 weeks post-conception. All ultrasound measurements, milestone tables, and screening windows are expressed in gestational weeks.
The most common recommendation is after week 12, when miscarriage risk drops from ~20% in the first trimester to less than 1%. Many couples share the news after seeing a heartbeat (weeks 6–8) with close family, then publicly announce after the 12-week scan. Some wait until after the 20-week anatomy scan, which screens for structural abnormalities. Ultimately, the timing is personal — some prefer early support if complications arise.
First trimester (10–13 weeks): NIPT (non-invasive prenatal testing, blood test) screens for chromosomal conditions; nuchal translucency ultrasound measures fluid behind fetal neck. Second trimester (15–20 weeks): quad screen blood test for chromosomal and neural tube risks; anatomy scan (18–22 weeks) checks organ development and placenta location. Third trimester: Group B strep test (36 weeks); monitoring for cervical changes and fetal position. Invasive tests (CVS, amniocentesis) are offered for high-risk results.