Free Pregnancy Week Calculator (+ Guide)
A free pregnancy week calculator that shows how far along you are, your trimester and what's developing, from any date you know.
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by Navid Moazzez·Updated Oct 3, 2026·5 min read·Navid.me is reader-supported. When you buy through links on this site, I may earn an affiliate commission. Learn more.You
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This free pregnancy week calculator tells you how many weeks pregnant you are, from the first day of your last period, your due date, your conception date or an IVF transfer date.
It shows your week and day, your trimester, how long is left, and what ACOG says is developing at your stage, for today or any day you pick.
I built it because "how many weeks pregnant am I?" has an exact answer, and it's counted from about 2 weeks before you conceived, which surprises many people the first time.
It's an estimate, not medical advice. Your doctor or midwife dates your pregnancy, usually with an early ultrasound.
TL;DRKey takeaways
What is a pregnancy week calculator?
A pregnancy week calculator works out how far along a pregnancy is on a given day, in weeks and days, the way doctors count it.
This one starts from whichever date you know, and shows the trimester, the days to your due date and what's developing that week.
How it works
The calculator works it out on your device, and your dates stay there.
How pregnancy weeks are counted
ACOG counts a pregnancy from the first day of your last period, and assumes it starts 2 weeks later. So those 2 extra weeks are part of the 40.
A pregnancy that's "17 and 3/7 weeks" is 17 weeks and 3 days along. At 17 weeks and 3 days you've finished 17 weeks, so you're in week 18.
The 3 trimesters
ACOG splits the 40 weeks into 3 trimesters.
- Weeks
- To 13 weeks and 6 days
- What happens
- Fertilization and the major organs develop
- Weeks
- 14 weeks to 27 weeks and 6 days
- What happens
- Rapid growth and development
- Weeks
- From 28 weeks
- What happens
- Weight goes on and the organs mature for life after birth
The calculator marks your day on a line through all 3.
If you know your due date
A due date from your doctor or an ultrasound is the best date you have, and it's what the calculator uses when you pick it.
It counts back 280 days to the first day of a last period, then counts your weeks forward from there.
If you had IVF
ACOG Committee Opinion 700 dates a pregnancy from IVF by the embryo's age and the transfer date.
- A day-5 embryo: the due date is 261 days after the transfer
- A day-3 embryo: the due date is 263 days after the transfer
Pick "IVF transfer", then the embryo's age, and the weeks follow from that due date.
What's happening week by week
These are ACOG's stretches of weeks, and what develops in each, in short.
| Weeks | What develops |
|---|---|
| Weeks 1 to 8 | The brain and spine begin to form, heart tissue starts to develop, and fingers and toes start to show on the hands and feet. |
| Weeks 9 to 12 | Cartilage for the arms and legs forms, the eyelids form but stay closed, the kidneys start making urine, and the fingernails form. |
| Weeks 13 to 16 | Bones harden, especially the long bones, the toenails form, hearing starts to develop, and the lungs begin forming the tissue for breathing. |
| Weeks 17 to 20 | The part of the brain that controls movement is fully formed, the digestive system is working, and soft hair called lanugo starts to cover the body. |
| Weeks 21 to 24 | Kicks and turns get stronger, the sucking reflex develops, fat forms under the skin, and the ridges that become fingerprints appear. |
| Weeks 25 to 28 | The lungs are formed but not ready yet and start making surfactant, the eyelids can open and close, and loud sounds can startle your baby. |
| Weeks 29 to 32 | Your baby can stretch, kick and grasp, the eyes sense changes in light, and the bone marrow is making red blood cells. |
| Weeks 33 to 36 | The bones harden while the skull stays soft, the fingernails reach the fingertips, and your baby may turn head-down for birth. |
| Weeks 37 to 40 | The circulation and the muscles and bones are done developing, the lungs, brain and nerves are finishing, and fat keeps building up. |
The calculator shows the stretch you're in.
A worked example
Here's the example the calculator loads: a last period that started 120 days ago.
120 days is 17 weeks and 1 day, so you're in week 18, in the second trimester. The due date is 280 days after that period started, 160 days from today, or 22 weeks and 6 days.
Mistakes to avoid
These are the slips that come up most.
- Counting from the day you conceived instead of the first day of your last period
- Reading "18 weeks pregnant" as week 18, when it's the start of week 19
- Keeping your period date after an ultrasound gives you a different due date
- Treating the due date as the day, when only about 1 in 20 babies arrive on it
Who is the pregnancy week calculator for?
It's for anyone who's pregnant, or supporting someone who is, and wants to know the week on any day, and these tools help around it.
- Due date calculator – for your due date and its milestones
- Ovulation calculator – for your fertile days
- Water intake calculator – for a day's water
- Sleep calculator – for when to go to bed
Bring your week to every appointment, and let your doctor's date be the one you go by.
Plan your health with your AI
Take your numbers further in
Pregnancy Week Calculator FAQs
Questions about the pregnancy week calculator? Here's what to know.
It works out how many weeks and days pregnant you are on a given day, counted the way doctors count, from the first day of your last period.
Count the days since the first day of your last period and divide by 7.
The calculator does it for today or any day you pick.
ACOG counts from the first day of your last period and assumes pregnancy starts 2 weeks later.
So the first 2 of the 40 weeks come before you're pregnant.
It means 17 weeks and 3 days of pregnancy, the way ACOG writes it.
No, at 18 weeks you've finished 18 weeks and you're in week 19.
The first runs to 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks, according to ACOG.
Yes, pick "Due date" and the calculator counts back 280 days to find your weeks.
ACOG sets the due date at 261 days after a day-5 embryo transfer, or 263 days after a day-3 one.
The weeks count back from that date.
It's as accurate as the date you give it.
ACOG says a first-trimester ultrasound is the most accurate way to date a pregnancy.
They stay in this browser and never reach a server.
Yes, it's free with no sign-up.
It runs in your browser, and your numbers stay on your device.
I did.
I'm Navid Moazzez, and I made the pregnancy week calculator as one of my free tools on navid.me.
More about me.
Navid Moazzez
AI business strategist & AI OS builder
Navid Moazzez helps creators and founders master AI and build their own AI Operating System (AI OS) to automate their business and life.
Navid.me is reader-supported. When you buy through links on this site, I may earn an affiliate commission. Read the affiliate disclaimer.
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