Pregnancy dating is based on your last menstrual period, not conception. Here's exactly how weeks and trimesters are calculated, what your due date means, and why scans sometimes change it.
Pregnancy is counted in weeks — but from a date most people find counterintuitive: the first day of your last menstrual period (LMP), not the day you conceived. Here's the logic behind the calculation and how to interpret what your midwife tells you.
Conception typically occurs around day 14 of a 28-day cycle — about two weeks after the LMP. Since the exact moment of conception is rarely known, and the LMP date usually is, medical professionals standardised on LMP dating. This means you're considered "2 weeks pregnant" even before conception has technically occurred.
Pregnancy apps and midwives always count this way. If you're told you're "8 weeks pregnant," that means 8 weeks since your last period — conception was approximately 6 weeks ago.
Only about 4% of babies are born on their exact due date. Around 80% are born within two weeks either side. The 40-week figure assumes a perfect 28-day cycle with ovulation on day 14 — which many people don't have.
Early ultrasound scans (before 14 weeks) are more accurate than LMP dating for establishing gestational age. If the scan date differs from LMP-calculated date by more than 5–7 days, your midwife will typically adjust your due date to match the scan measurement.
If your cycle is regularly longer or shorter than 28 days, your actual ovulation date — and therefore conception — differs from the textbook day 14:
This adjustment is built into most pregnancy calculator apps. The early dating scan will confirm or refine this regardless.
The medical definitions have shifted in recent years:
Babies born at 37 weeks were previously called "full term" but are now recognised as having slightly higher rates of breathing difficulties and feeding challenges than those born at 39–40 weeks. The medical community has refined guidance accordingly.