Pregnancy weeks and trimesters: how dating works
6 min read · Updated 2026-10-11 · FitCalc
Pregnancy is measured in weeks from the first day of your last menstrual period, not from the moment of conception. That convention can make the numbers confusing at first, but once understood it makes the whole timeline — trimesters, milestones and the due date — much clearer. This is general information, not a substitute for professional care.
Why pregnancy is dated from the last period
The exact date of conception is rarely known, whereas the first day of the last menstrual period is a date most people can recall. Pregnancy dating therefore uses that as the starting point, which means roughly the first two weeks of a '40-week' pregnancy occur before conception actually takes place.
A full-term pregnancy is usually described as 40 weeks from that starting point. Because the true ovulation date varies between people and between cycles, this is a convention rather than a precise measure of development.
The three trimesters
The first trimester runs from the start of pregnancy to about the end of week 13. It is the period of the most rapid development, and it is when many people experience fatigue and nausea. The second trimester, roughly weeks 14 to 27, is often the period when symptoms ease and energy returns.
The third trimester covers about week 28 until birth. It brings a growing physical load, more frequent appointments and preparation for delivery. The boundaries are approximate, and providers may describe them slightly differently.
- First trimester — up to about week 13
- Second trimester — about weeks 14 to 27
- Third trimester — about week 28 to birth
- Full term — around 39 to 40 weeks from the last period
Why the due date is an estimate
The standard due date is calculated by adding 280 days (40 weeks) to the first day of the last period, and it is refined by an early ultrasound where available. Only a small proportion of babies arrive exactly on the estimated date.
Providers usually talk about a range around the due date rather than a single day. A baby born within a few weeks either side can still be within normal, though earlier birth may require additional support.
Tracking your own weeks
Knowing your current week and trimester is useful for understanding what to expect and when appointments typically fall. A pregnancy week calculator can show the current week, trimester and key milestone dates from either your last period or a known conception date.
If a provider has given you a dating based on ultrasound, that dating usually supersedes a calculated estimate, because early scans measure the pregnancy directly.
When to seek professional advice
This guide is general educational information only. Any concerns about symptoms, growth, or the timing of care should be raised with a qualified healthcare professional, who can assess your individual situation.
Antenatal care is individualised for good reason: factors such as medical history, multiple pregnancy and previous complications all change the appropriate plan.
Run your own numbers
Free calculators that answer the “how much” version of this question. They run entirely in your browser — nothing you type is uploaded or stored.
Common questions
Why does pregnancy start before conception?
Because dating begins from the first day of the last menstrual period, roughly two weeks before ovulation and conception typically occur. It is a dating convention, not a claim about when development began.
How accurate is an ultrasound dating?
Early ultrasound dating is generally more accurate than a calculation from the last period, because it measures the pregnancy directly. Later scans are less precise for dating.
FitCalc provides general information only and is not medical advice, diagnosis or treatment. Figures are illustrative and depend on your own circumstances. Always consult a qualified health professional about your situation. See our full disclaimer.