Pregnancy Details
Select the first day of your last period
How weeks are counted
Pregnancy is dated from the first day of your last period (LMP). Your "weeks pregnant" is the number of completed weeks since then — about 2 weeks more than the actual age of the baby.
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Enter a date above to see how many weeks pregnant you are
Weeks to Months Quick Guide
| Month | Weeks | Trimester |
|---|---|---|
| Month 1–3 | 1–13 | First |
| Month 4–6 | 14–27 | Second |
| Month 7–9 | 28–40 | Third |
Months and weeks don't line up exactly because months vary in length. Doctors always track pregnancy in weeks, which is more precise.
Key Milestones by Week
- →Week 6–8 — first ultrasound; baby's heartbeat can be detected.
- →Week 12 — end of the first trimester; risk of miscarriage drops sharply.
- →Week 18–20 — anomaly scan checks the baby's organs and growth.
- →Week 24 — viability milestone; baby has a chance of survival if born.
- →Week 37 — baby is considered early term and ready for birth.
How Gestational Age Is Calculated
Your gestational age is simply the time since your last period, expressed in weeks and days:
Days pregnant = Today − LMP
Weeks = whole weeks in those days · Days = remainder
Due date = LMP + 280 days (40 weeks)
Weeks = whole weeks in those days · Days = remainder
Due date = LMP + 280 days (40 weeks)
Example
If your LMP was 100 days ago, you are 100 ÷ 7 = 14 weeks and 2 days pregnant — in your second trimester.
Frequently Asked Questions
Common questions about pregnancy weeks and gestational age
How do I know how many weeks pregnant I am?
Pregnancy is counted in weeks from the first day of your last menstrual period (LMP), not from conception. Count the number of days from your LMP to today and divide by 7. The whole number is your completed weeks and the remainder is extra days — for example, 14 weeks and 3 days. This calculator does it instantly from your LMP, due date, or conception date.
Why is pregnancy counted from the last period?
The first day of your last period is an easy, observable date, whereas the exact moment of conception is usually unknown. Conception happens around 2 weeks after the LMP, so a 40-week pregnancy from LMP equals about 38 weeks of actual foetal age. This LMP-based convention is used by doctors worldwide for consistency.
How many weeks are in each trimester?
The first trimester is weeks 1–12, the second trimester is weeks 13–26, and the third trimester is weeks 27–40. A full-term pregnancy lasts about 40 weeks from the LMP, though delivery anywhere between 37 and 42 weeks is considered normal.
How accurate is the week calculation?
It is accurate if you remember your LMP and have regular cycles. For irregular cycles, the estimate can be off by several days. A first-trimester dating ultrasound (measuring crown-rump length) is the most accurate way to confirm gestational age, and your doctor may adjust the dates based on it.
How do pregnancy weeks convert to months?
As a rough guide: month 1 is weeks 1–4, month 2 weeks 5–8, month 3 weeks 9–13, month 4 weeks 14–17, month 5 weeks 18–22, month 6 weeks 23–27, month 7 weeks 28–31, month 8 weeks 32–35, and month 9 weeks 36–40. Doctors track weeks because they are more precise than months.
What is gestational age?
Gestational age is how far along your pregnancy is, measured from the first day of your last menstrual period. It is expressed in weeks and days (e.g. 22 weeks 4 days). It differs from foetal age, which is measured from conception and is about 2 weeks less.