How to use the pregnancy calculator
Pick the dating method that matches the best information you have, enter the dates, and choose the day you want to check. It starts on today's date, but you can pick any past or future day to see how far along you'll be.
If your doctor or midwife has already given you a due date, use the Known due date option. ACOG advises that once a due date is set, changes to it should be discussed and recorded by your care team, so that date is the one to plan around.
- Choose a method: last menstrual period, known due date, conception date, ultrasound gestational age, or IVF embryo transfer.
- Enter the date for that method. For your last period, use the first day bleeding started and your usual cycle length.
- For an ultrasound, enter the scan date and the weeks and days printed on the report. For IVF, enter the transfer date and whether it was a day-3 or day-5 embryo.
- Keep today's date or pick another date to check.
- Press Calculate to see your due date, weeks and days, trimester and the dating timeline.
Pregnancy weeks calculator example
Every method lands on the same clock. The examples below start from four different records. All of them give a due date of January 6, 2027.
On July 9, 2026, each of them reports 14 weeks and 1 day and the second trimester. On October 5, 2026, the same pregnancy is 26 weeks and 5 days along, with 93 days to go.
| Dating method | What you enter | Due date | What it assumes |
|---|---|---|---|
| Last period | April 1, 2026; 28-day cycle | January 6, 2027 | Ovulation near cycle day 14 |
| Known conception | April 15, 2026 | January 6, 2027 | You know the conception or ovulation date |
| Ultrasound report | 14w1d on July 9, 2026 | January 6, 2027 | Weeks and days come from the report |
| Day-5 IVF transfer | April 20, 2026 | January 6, 2027 | Embryo was five days old at transfer |
| Known due date | January 6, 2027 | January 6, 2027 | Your care team's date is used as is |
How the due date is calculated
The standard rule adds 280 days (40 weeks) to the first day of your last period. The calculator also shifts that date by how far your usual cycle is from 28 days. With a 32-day cycle and the same April 1 period, the due date moves four days later, to January 10, 2027.
A known conception date gets 266 days, because the gestational clock starts about two weeks before conception. IVF dates follow ACOG's examples: 261 days after a day-5 transfer and 263 days after a day-3 transfer.
Last period: due date = first day of last period + 280 days + (cycle length - 28 days)Known conception: due date = conception date + 266 daysUltrasound: due date = scan date + 280 days - reported gestational age in daysDay-5 IVF transfer: due date = transfer date + 261 daysDay-3 IVF transfer: due date = transfer date + 263 days
Trimester and term dates
The calculator lists the dates each stage starts, based on the same timeline. Trimester boundaries follow ACOG's patient guidance, and the term labels follow ACOG's definition of term pregnancy. These labels describe timing. They aren't advice about when delivery should happen.
Here's the timeline for a last period of April 1, 2026 with a 28-day cycle.
| Milestone | Gestational age | Date |
|---|---|---|
| Estimated conception | 2 weeks 0 days | April 15, 2026 |
| Second trimester begins | 14 weeks 0 days | July 8, 2026 |
| Third trimester begins | 28 weeks 0 days | October 14, 2026 |
| Early term begins | 37 weeks 0 days | December 16, 2026 |
| Full term begins | 39 weeks 0 days | December 30, 2026 |
| Estimated due date | 40 weeks 0 days | January 6, 2027 |
| Late term begins | 41 weeks 0 days | January 13, 2027 |
| Postterm begins | 42 weeks 0 days | January 20, 2027 |
When can you take a pregnancy test?
The Office on Women's Health says most home tests can detect pregnancy accurately after a missed period, and some more sensitive tests work a little earlier. If you test too soon and get a negative result, testing again after your period is due is more reliable.
With a 28-day cycle, your next period is due 28 days after the last one started. That's when you'd be 4 weeks 0 days pregnant on the gestational clock. This calculator doesn't predict a test date, but the LMP method shows you that 4-week mark if you set the check date to it.
Which dating method is most accurate?
ACOG calls a first-trimester ultrasound the most accurate way to set or confirm gestational age. For pregnancies from IVF or other assisted reproduction, it recommends using the treatment dates. Your last period works well when you're sure of the date and your cycles are regular.
Irregular cycles, recent hormonal birth control, a recent birth, or uncertain bleeding make the last-period estimate less dependable. The cycle-length setting helps a little, but it can't see when you actually ovulated.
What this calculator can't tell you
This is a calendar tool. It can't confirm a pregnancy, check on the baby's growth, read an ultrasound image or tell you if a symptom is normal. ACOG notes that only about 1 in 20 women give birth on their due date, so treat it as a reference point.
Start prenatal care early. CDC lists urgent warning signs that need care right away. They include trouble breathing, chest pain, fainting, a severe headache that won't go away, severe belly pain, fever, heavy bleeding or fluid leaking, and thoughts of harming yourself or your baby.
Sources
These clinical and public-health references back up the dating formulas, trimester and term boundaries, test timing and warning signs on this page.