How Pregnancy Weeks Work —
And Why You Count From Your Period

The first thing that confuses almost everyone about pregnancy is the maths. Your doctor says you're "eight weeks pregnant" when you only found out days ago — and the count apparently started two weeks before you conceived. It isn't a mistake. Once you understand the logic, every scan, milestone and due-date conversation makes sense.

How to Work Out How Many Weeks You Are

The rule is simple: count the days from the first day of your last menstrual period (LMP) to today, then divide by seven. The whole number is your completed weeks; the leftover is the extra days.

So if your last period began 100 days ago, you are 14 weeks and 2 days pregnant — written as 14+2 in medical shorthand.

Why Count From the Last Period at All?

It seems strange to start the clock before pregnancy exists. There are two good reasons:

• The LMP is a date you actually know. Almost everyone can name the first day of their last period. Hardly anyone knows the exact hour of conception.
• It creates a single, shared standard. By always dating from the LMP, every doctor, scan and pregnancy app speaks the same language — 40 weeks from the LMP, everywhere.

Conception actually occurs around ovulation, roughly two weeks into the cycle. So those "first two weeks" of pregnancy are really the run-up to conception. It's a quirk of the convention, not a measure of the baby's true age.

Gestational Age vs Fetal Age

This is the distinction that clears up all the confusion:

• Gestational age — measured from the LMP. This is what "weeks pregnant" means and what doctors use.
• Fetal age (or conceptional age) — measured from conception. About two weeks less.

So at 12 weeks gestational age, the developing baby is really about 10 weeks old. Both are correct; they just count from different starting lines. When anyone says "weeks pregnant," they mean gestational age.

The Three Trimesters

Weeks to Months — the Tricky Conversion

People think in months; doctors think in weeks. They don't divide neatly because a month is longer than four weeks. Here's the working guide:

The Scans and Milestones That Matter

• Dating scan (6–12 weeks) — confirms the heartbeat and sets the most accurate due date.
• NT scan (11–14 weeks) — screens for certain chromosomal conditions.
• Anomaly scan (18–20 weeks) — a detailed check of the baby's organs and growth.
• Viability (24 weeks) — the point from which a baby has a real chance of surviving if born early.
• Term (37–42 weeks) — the safe window for birth; 39–40+6 is "full term."

How Accurate Is Your Due Date?

Treat the due date as the middle of a range, not a deadline. Only about 5% of babies arrive on the exact date — most are born any time from 37 to 42 weeks. The most accurate dating comes from a first-trimester ultrasound, which measures the baby directly; your doctor may shift an LMP-based estimate to match it. If your cycles are irregular or you don't remember your LMP, that early scan becomes the primary way to date the pregnancy. For IVF, the dates are calculated from the transfer day and are more precise still.

Once you know your LMP, the Pregnancy Week Calculator tells you your exact week and trimester today, and the Due Date Calculator maps the whole journey ahead.

This article is general information, not medical advice. For personal guidance about your pregnancy, consult a qualified doctor or gynaecologist.