What does "weeks pregnant" mean?
You see two lines on the test, and the first question is almost always the same: how far along am I? The answer can feel confusing because "week 4" doesn't mean you've been pregnant for four weeks. Conception usually happens about two weeks after your last period starts, so your first two "pregnant" weeks happen before you even conceive.
This calculator turns whatever date you have into your current week and day, your trimester and your due date. Using IVF? Our IVF due date calculator counts from your transfer or retrieval date instead.
How we count your weeks of pregnancy
We find your "week 0 day 0", then count the days since. Which date you start from depends on what you know.
| What you enter | How we find day 0 | Due date |
|---|---|---|
| First day of last period | Your last period date | Last period + 280 days (adjusted if your cycle isn't 28 days) |
| Due date | Due date minus 280 days | The date you entered |
| Conception date | Conception date minus 14 days | Conception + 266 days |
If your cycle runs longer or shorter than 28 days, ovulation shifts with it, so we add or subtract the difference. A 32-day cycle, for example, moves your due date four days later.
Pregnancy trimesters by week
Pregnancy has three trimesters, and your week number tells you which one you're in. The first trimester ends at 13 weeks and 6 days, the second at 27 weeks and 6 days, and the third runs until baby arrives.
| Trimester | Weeks | What often happens |
|---|---|---|
| First | 1 to 13 | Baby's organs form. Many moms feel tired and nauseous. Your first prenatal visits happen here. |
| Second | 14 to 27 | Energy often returns. You may feel the first kicks and have your anatomy scan. |
| Third | 28 to 40 | Baby gains weight fast. This is the time to line up your hospital bag and your breast pump. |
Weeks to months
Pregnancy lasts about 9 months, but weeks don't divide evenly into months. This is the usual conversion.
| Month | Weeks | Month | Weeks |
|---|---|---|---|
| 1 | 1 to 4 | 6 | 23 to 27 |
| 2 | 5 to 8 | 7 | 28 to 31 |
| 3 | 9 to 13 | 8 | 32 to 35 |
| 4 | 14 to 17 | 9 | 36 to 40 |
| 5 | 18 to 22 |
When to trust your last period vs your ultrasound
Your last period gives a solid first estimate, and an early ultrasound can refine it. Doctors measure the baby, compare the result with your dates, and change your due date only when the gap is large enough.
Stick with your last period date when
- Your cycles are regular and close to 28 days
- Your early ultrasound lands within about 5 to 7 days of your date
- You know the exact day your last period started
Expect your doctor to adjust it when
- Your cycles are irregular or you can't remember the date
- The first-trimester ultrasound differs by more than about a week
- You conceived while breastfeeding or after stopping birth control
When your provider gives you a due date, use that one. You can enter it above under "Due date" to see your weeks and your pump window.
When to order your breast pump
Most moms can order a breast pump 30 to 60 days before the due date, and the calculator above shows your exact window. Under the Affordable Care Act (ACA), most health plans cover a breast pump, and most Hygeia moms pay $0 out of pocket. Our insurance coverage page explains the details.
Ordering early takes pressure off your last weeks. Insurance plans vary on timing, and some need a prescription first. Our team contacts your doctor's office for you, so you can focus on the rest of your to-do list.
Check your coverage while you're thinking about it
It takes about 2 minutes. We verify your benefits, contact your doctor and ship a hospital-grade pump to your door.
Your next steps
Weeks pregnant calculator FAQ
How many weeks pregnant am I?
Enter the first day of your last period above and the calculator counts the full weeks and days since then. If you only know your due date or conception date, switch the tab and we'll work backward. Your doctor's ultrasound date always takes priority.
Do doctors count pregnancy from conception or from the last period?
Doctors count from the first day of your last period. Conception usually happens about two weeks later, so a baby who is 38 weeks old from conception is called 40 weeks pregnant. This standard makes dates consistent across providers.
How many weeks is 9 months of pregnancy?
Nine months equals about 40 weeks, or roughly 280 days. Months vary between 28 and 31 days, so weeks give a more precise count. Most providers talk in weeks for that reason.
When does the second trimester start?
The second trimester starts at week 14 and runs through week 27. The first trimester ends at 13 weeks and 6 days. Many moms notice more energy and less nausea around this time.
When does the third trimester start?
The third trimester starts at week 28 and lasts until delivery. It's a good time to pack your hospital bag, choose your pediatrician and check your breast pump coverage.
How accurate is a due date from the last period?
It's a solid estimate if your cycles are regular, but only about 1 in 20 babies arrives on the exact date. Most are born between 37 and 41 weeks. An early ultrasound can refine the date if your cycle is irregular.
Can I use this calculator if I had IVF?
You can enter your estimated due date under the Due date tab, but IVF dating works differently. Use our IVF due date calculator to count from your embryo transfer or egg retrieval date, which gives a more precise answer.
When should I order my breast pump?
Most insurance plans let you order 30 to 60 days before your due date, and the calculator shows your exact window. Starting early leaves time for the prescription and delivery. Check your eligibility on our insurance form, which takes about 2 minutes.
Sources and review
- American College of Obstetricians and Gynecologists (ACOG), Committee Opinion No. 700: Methods for Estimating the Due Date.
- ACOG, Committee Opinion No. 579: Definition of Term Pregnancy.
- Centers for Disease Control and Prevention (CDC): Vaccines during pregnancy, Tdap recommendation.
- Health Resources and Services Administration (HRSA), Women's Preventive Services Guidelines: breastfeeding services and supplies.
This page is educational and doesn't replace advice from your doctor or midwife. Insurance coverage varies by plan, and *$0 applies to most plans.