Exactly how far along are you?

How weeks are counted

Gestational age is counted from the first day of your last menstrual period (LMP), regardless of when conception happened. This is the obstetric convention used in Canada — “28 weeks pregnant” means 28 weeks since LMP, not 28 weeks since conception. This is why the first two weeks of a pregnancy are technically before fertilisation.

The big milestone weeks

• Week 8 — embryo becomes a fetus.
• Week 13 — end of the first trimester.
• Week 18–22 — anatomy ultrasound usually offered.
• Week 24 — “viability” threshold begins (with NICU support).
• Week 27 — end of the second trimester.
• Week 37 — full term begins.
• Week 40–41 — most healthy pregnancies deliver in this window.

Calendar dating vs ultrasound dating

First-trimester ultrasound is more accurate than calendar dating for women with irregular cycles or unknown LMP. If the ultrasound disagrees with the LMP-calculated due date by more than ~5–7 days, your provider will use the ultrasound date. This calculator uses the calendar method, which is the right tool for tracking week-by-week progress between visits.