How Many Weeks Pregnant Am I?
Enter the first day of your last period, your due date or your conception date. This free calculator tells you how many weeks and months pregnant you are, which trimester you are in, and roughly how big your baby is this week.
How pregnancy weeks are counted
Doctors count pregnancy from the first day of your last period, not from the day you conceived. This is called gestational age. Because conception usually happens about two weeks after your period starts, you are already counted as "2 weeks pregnant" on the day you conceive. A full pregnancy is 40 weeks, or 280 days, from your last period.
The calculator works out your gestational age like this:
| If you know your | Pregnancy start date is |
|---|---|
| Last period | The first day of that period |
| Due date | Due date minus 280 days |
| Conception date | Conception date minus 14 days |
Example: if your last period started on March 1 and today is June 14, 105 days have passed. That is 15 weeks 0 days, month 4, in the second trimester.
Pregnancy weeks to months
Forty weeks does not divide neatly into nine months, so guides group the weeks like this:
| Month | Weeks | Trimester |
|---|---|---|
| Month 1 | 1 – 4 | First |
| Month 2 | 5 – 8 | |
| Month 3 | 9 – 13 | |
| Month 4 | 14 – 17 | Second |
| Month 5 | 18 – 22 | |
| Month 6 | 23 – 27 | |
| Month 7 | 28 – 31 | Third |
| Month 8 | 32 – 35 | |
| Month 9 | 36 – 40 |
The three trimesters
First trimester (weeks 1 to 13): the baby's organs form. Nausea, tiredness and sore breasts are common. This is the most important time to take folic acid and avoid alcohol and smoking.
Second trimester (weeks 14 to 27): often the most comfortable stage. Your bump starts to show, energy returns and you will probably feel the baby move for the first time.
Third trimester (weeks 28 to birth): the baby gains most of its weight. Backache, heartburn and poor sleep are common, and prenatal visits become more frequent.
Baby size by week
| Week | About the size of | Approximate weight |
|---|---|---|
| Week 8 | A raspberry | under 5 g |
| Week 12 | A lime | about 14 g |
| Week 16 | An avocado | about 100 g |
| Week 20 | A banana | about 300 g |
| Week 24 | An ear of corn | about 600 g |
| Week 28 | An eggplant | about 1 kg |
| Week 32 | A squash | about 1.7 kg |
| Week 36 | A honeydew melon | about 2.6 kg |
| Week 40 | A watermelon | about 3.4 kg |
These are rough averages. Every baby grows at its own pace, and your doctor will check growth at your scans.
Frequently asked questions
How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by 7. The whole number is your weeks and the remainder is your days. The calculator above does this for you.
How many months pregnant is 20 weeks?
20 weeks is in month 5, the middle of the second trimester, and exactly halfway through a 40-week pregnancy.
When does the second trimester start?
The second trimester starts at 14 weeks and runs until the end of week 27. The third trimester starts at 28 weeks.
Why am I 2 weeks pregnant before I even conceived?
Because pregnancy is dated from your last period, which usually starts about two weeks before ovulation and conception. This gives everyone a clear, shared starting date.
Is pregnancy 9 months or 10 months?
Both are heard. 40 weeks is a little over 9 calendar months, which is why some people say 10 lunar months of 4 weeks each.