Weeks, trimesters and months
| Trimester | Weeks | Roughly |
|---|---|---|
| First | 1 – 13 | Months 1–3 |
| Second | 14 – 27 | Months 4–6 |
| Third | 28 – 40+ | Months 7–9 |
Pregnancy weeks are “gestational age”: counted from the start of the last period, about two weeks before conception. That’s why an embryo’s age (from conception) is two weeks less than the pregnancy week.
Your midwife or doctor may move your dates after an early ultrasound; if so, use their due date in the box above. Any bleeding, severe pain, or a change in baby’s movements needs a call to your care team straight away.
Questions people ask
How far along am I?
Count from the first day of your last period: the number of whole weeks since then is how many weeks pregnant you are. So two weeks after conception you are already about four weeks pregnant by medical convention.
How many weeks is each trimester?
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Some sources split them slightly differently.
How many months pregnant is 20 weeks?
Around 4½ months. Pregnancy is about 40 weeks, which is a little over nine calendar months, so weeks don’t convert neatly into months.
Why do doctors count from the last period?
Because most people know when their period started but not exactly when they ovulated or conceived. Counting from the period gives everyone the same starting point, later refined by ultrasound.
Sources
- ACOG Committee Opinion No. 700. Methods for estimating the due date.
- ACOG Committee Opinion No. 579. Definition of term pregnancy (early, full, late and post-term).
- Mayo Clinic. Fetal development week by week.
Every formula on VitalMetrics is listed with its original reference on Sources & methods.