How is a pregnancy due date calculated?
Naegele's Rule is the standard method: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a regular 28-day cycle with ovulation on day 14. For cycles longer or shorter than 28 days, the due date shifts accordingly — add the difference. An early ultrasound (before 14 weeks) provides the most accurate dating based on foetal measurements (Crown-Rump Length).
How accurate is the estimated due date (EDD)?
Very few babies arrive exactly on their EDD — only about 4–5%. About 80% of babies are born within 2 weeks either side. The safe "full term" window is 37–42 weeks; births before 37 weeks are preterm. After 41 weeks, healthcare providers typically recommend induction. The EDD is a planning tool, not a precise prediction.
What are the three trimesters of pregnancy?
First trimester: weeks 1–12 — organ formation, most sensitive period for birth defects, miscarriage risk highest. Second trimester: weeks 13–27 — often the most comfortable, baby's sex visible on scan around week 20, quickening (first movements) typically felt at weeks 16–20. Third trimester: weeks 28–40+ — rapid growth, baby's position shifts, lungs mature around 34–36 weeks.
What key milestones should I expect during pregnancy?
Week 4–5: positive pregnancy test. Week 8–12: first prenatal appointment, heartbeat detected by ultrasound. Week 12: nuchal translucency (Down syndrome) scan. Week 16–20: anatomy scan, sex can be confirmed. Week 24: viability threshold (survival outside womb possible with intensive care). Week 28: third trimester begins, gestational diabetes screening. Week 36–37: weekly monitoring begins.
Can I calculate due date from conception date or ultrasound?
Yes. From conception: add 266 days (38 weeks) rather than 280 — because conception typically occurs 14 days after LMP. From ultrasound dating: your sonographer calculates gestational age from the Crown-Rump Length (CRL) in the first trimester, or head/abdominal circumference in the second trimester. NHS and ACOG recommend using the first ultrasound date if it differs by more than 7 days from LMP dating.