Pregnancy Week Calculator
This pregnancy week calculator tells you how far along you are in weeks and days, which trimester you are in, your estimated due date and how many days are left. Start from the first day of your last menstrual period (LMP), a due date you already have, the date of conception or an IVF embryo transfer. It is an estimate: an early ultrasound gives the most accurate dating.
Updated
Results
- Estimated due date40 weeks (280 days) from the equivalent last period
- Days until due dateNegative if the due date has passed
- Estimated conception dateAbout 14 days after the start of the last period
- Pregnancy progressShare of 40 weeks completed
Results are estimates. See the methodology below.
How to use the calculator
- Choose what you know: last period, due date, conception date, or a day-3 or day-5 embryo transfer.
- Enter that date. For the last-period method, use the first day of bleeding, not the last.
- If your cycle is usually longer or shorter than 28 days, open Advanced options and enter your average cycle length in days (21 to 45).
- Leave the second date as today, or pick a future day, such as an appointment.
- Read the result: completed weeks and days, trimester, due date, days remaining, estimated conception date and the share of a 40-week pregnancy already behind you.
How gestational age is calculated
Doctors and midwives count pregnancy from the first day of the last menstrual period, not from conception, so you are already about two weeks pregnant when conception happens. A full-term pregnancy is counted as 280 days (40 weeks) from the LMP, the rule of thumb known as Naegele’s rule.
The calculator first converts whatever you enter into an equivalent LMP date:
| Method | Equivalent LMP |
|---|---|
| Last period, 28-day cycle | The date you enter |
| Last period, other cycle length | Shifted by (cycle − 28) days, because ovulation happens later in longer cycles |
| Due date | Due date − 280 days |
| Conception | Conception − 14 days |
| Day-3 embryo transfer | Transfer − 17 days |
| Day-5 embryo transfer | Transfer − 19 days |
The day-5 rule gives a due date 261 days after the transfer, and the day-3 rule 263 days after, as described by the American College of Obstetricians and Gynecologists (ACOG). The number of days between the LMP and the chosen date is then split into completed weeks plus days. Trimesters follow the usual boundaries: first up to 13 weeks 6 days, second from 14 weeks 0 days to 27 weeks 6 days, third from 28 weeks 0 days. The estimated conception date is LMP + 14 days; for IVF it matches the egg retrieval and fertilization day.
Examples
Last period on July 6, 2026, regular 28-day cycle
On October 12, 2026 you are 14 weeks 0 days, just into the second trimester. Estimated due date: April 12, 2027, 182 days away. Estimated conception: July 20, 2026. Progress: 35%.
Same last period, 32-day cycle
Ovulation is assumed four days later, so on October 12, 2026 you are 13 weeks 3 days, still first trimester. The due date moves to April 16, 2027, with 186 days to go.
Day-5 embryo transfer on August 20, 2026
On October 12, 2026 you are 10 weeks 2 days. Due date: May 8, 2027, 208 days away. Fertilization is dated to August 15, 2026, and progress is 26%.
Reading your result
A due date is a midpoint, not an appointment. The NHS notes that pregnancy normally lasts from 37 to 42 weeks from the first day of the last period, a five-week window around the date shown here.
Ultrasound dating takes precedence. ACOG considers a first-trimester ultrasound measuring crown-rump length (up to 13 weeks 6 days) the most accurate way to date a pregnancy. If the ultrasound differs from your LMP date by more than 5 days before 9 weeks, or more than 7 days between 9 weeks 0 days and 13 weeks 6 days, your clinician will use the ultrasound date. Once your due date is set at a prenatal visit, enter it with the due-date method so that your week count matches your chart. For IVF pregnancies, the date based on the transfer is used.
Days remaining turn negative after the due date. Results outside 0 to 44 weeks show an error.
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 completed weeks and the remainder is days, so 98 days is 14 weeks 0 days. The calculator does this for you.
Why does pregnancy count two weeks before conception?
Because the first day of a period is a date most people know, while ovulation and conception usually can’t be pinpointed. With a 28-day cycle, ovulation happens about 14 days after the period starts, so those two weeks are part of the count.
When does the second trimester start?
At 14 weeks 0 days. The first trimester runs through 13 weeks 6 days, and the third trimester starts at 28 weeks 0 days.
My ultrasound due date is different from the calculator. Which is right?
Use the date your doctor or midwife gives you. An early ultrasound measures the embryo directly and corrects for late ovulation or an uncertain period date. Enter that due date with the due-date method.
How is the due date calculated after IVF?
From the transfer date and the embryo’s age: 261 days after a day-5 (blastocyst) transfer and 263 days after a day-3 transfer. Both correspond to 280 days from an equivalent LMP.
What if my cycles are irregular?
The last-period method assumes regular cycles. With irregular cycles or a recently stopped birth control pill, the estimate can be off by weeks, and an ultrasound is the reliable way to date the pregnancy.
Sources
This calculator gives an estimate for general information and is not medical advice. Your doctor or midwife will confirm your due date, usually with an ultrasound, and that date takes precedence.