How your due date is calculated
Most pregnancy calculators use Naegele's Rule, the standard obstetric method for estimating due date (EDD — Estimated Delivery Date):
Alternatively, add 1 year to your LMP, subtract 3 months, and add 7 days. Both methods give the same result.
Why 40 weeks? Pregnancy is dated from the first day of your last menstrual period — approximately 2 weeks before conception actually occurs. So the "40 weeks" of pregnancy includes the 2 weeks before fertilisation. Actual time from conception to birth is approximately 38 weeks.
How accurate is an EDD?
Only about 4–5% of babies are born on their exact due date. The majority of births occur within a 2-week window around the EDD — between weeks 39 and 41. A pregnancy is considered full-term at 39–40 weeks, early term at 37–38 weeks, and late term at 41 weeks. Going past 42 weeks is considered post-term, at which point induction is typically discussed.
The EDD becomes more accurate when confirmed or adjusted by an early ultrasound (before 14 weeks), which can measure the foetus and refine the date. If ultrasound dating differs from LMP dating by more than 7 days (first trimester), the ultrasound date is typically used.
Pregnancy trimesters: dates and what happens
First trimester: Weeks 1–12
The embryo develops all major organs and structures. Morning sickness, fatigue, and breast tenderness are common. The miscarriage risk is highest in this period — approximately 10–20% of known pregnancies. The first antenatal appointment (booking appointment) and nuchal translucency ultrasound typically occur in weeks 10–13.
Second trimester: Weeks 13–26
Often considered the most comfortable trimester. The foetus grows rapidly and movements become noticeable (quickening) around weeks 18–22. The anatomy (anomaly) scan occurs at weeks 18–21. For most women, nausea improves and energy returns.
Third trimester: Weeks 27–40+
The foetus gains most of its birth weight. The mother may experience Braxton Hicks contractions, back pain, and shortness of breath as the uterus presses on the diaphragm. The baby's position becomes important from 36 weeks — most will turn head-down (cephalic) by this point.
Knowing your exact week count matters for prenatal care
Specific prenatal tests, scans, and interventions are timed precisely by week:
- Week 8–10: Non-invasive prenatal testing (NIPT) can be offered for chromosomal conditions
- Week 10–13: Chorionic villus sampling (CVS) if indicated
- Week 15–20: Amniocentesis if indicated
- Week 28: Glucose tolerance test for gestational diabetes; Rh-negative mothers receive Anti-D injection
- Week 36: Group B streptococcus (GBS) testing (US practice; timing varies by country)
- Week 41: Membrane sweep typically offered; induction discussed
Pregnancy weight gain: IOM guidelines by pre-pregnancy BMI
- Underweight (BMI under 18.5): 28–40 lbs (12.7–18.1 kg) recommended
- Normal weight (BMI 18.5–24.9): 25–35 lbs (11.3–15.9 kg)
- Overweight (BMI 25–29.9): 15–25 lbs (6.8–11.3 kg)
- Obese (BMI 30+): 11–20 lbs (5–9.1 kg)
These guidelines come from the Institute of Medicine (IOM) and are the standard reference in the US, Canada, Australia, and the UK. Always discuss your personal weight gain target with your midwife or OB/GYN, as individual circumstances vary.