Pregnancy Calculator

Estimate a due date from the last period, a conception date, or an ultrasound. The result names which rule ran. An estimate, not prenatal care.

Date Format

Calculation Method *

Last Menstrual Period Date *

Smart Tips

• Last menstrual period: due date is that date plus 280 days (Naegele, 40 weeks).

• Conception: due date is that date plus 266 days.

• Ultrasound: due date is the scan date plus 280 minus gestational days (weeks × 7 + days, days 0–6).

• Completed weeks start at 0. First trimester is weeks 0–12, second is 13–26, and third is week 27 or later.

• A start date still ahead is labeled Before week 0.

• This is a date estimate, not prenatal care.

The Formula

LMP due date = LMP + 280 days
Conception due date = conception + 266 days
Ultrasound due date = scan date + (280 − gestational days)

280 days = LMP rule and ultrasound countdown
Naegele, 40 weeks from the period date

266 days = Conception rule
280 minus 14 days

Weeks = LMP: days since LMP ÷ 7, rounded down. Conception: add 14 days first. Ultrasound: (weeks × 7 + days + days since the scan) ÷ 7.
Trimester labels: weeks 0–12, 13–26, then week 27 or later

Understanding Pregnancy

First Trimester

Completed weeks 0–12

Page label: First trimester when the completed week is under 13, including week 0.

• This is the week-count label, not a list of prenatal visits.

Second Trimester

Completed weeks 13–26

Page label: Second trimester when the completed week is 13 through 26.

• Week 26 is still the second label. Week 27 switches to the third.

Third Trimester

Completed week 27 or later

Page label: Third trimester from completed week 27 onward, including week 41 and later.

• The label does not stop at week 40.

Key Insights

280 vs 266

280 days is added to the last menstrual period, and it is also the countdown used after an ultrasound. 266 days is added to a conception date. The result states which rule ran.

Completed weeks

The LMP date is week 0. Conception weeks add 14 days before dividing by 7. Ultrasound weeks start from the age you enter.

Trimester labels

Weeks 0–12, 13–26, and week 27 or later. A future start date is Before week 0. These are labels for the count, not a care schedule.

Not prenatal care

The date is an estimate from the rule above. This page does not provide prenatal care or predict the birth date.

Medical Disclaimer

Estimate only: This page adds 280 or 266 days, or counts forward from an ultrasound age you type. It is not prenatal care, a diagnosis, or a prediction of when a birth will happen.

Not a care plan: Trimester labels follow the completed-week count on this page. They are not screening advice or a visit schedule.

Frequently Asked Questions

The label under the date names the rule for the method you selected. Last menstrual period adds 280 days (Naegele, 40 weeks). Conception adds 266 days. Ultrasound adds 280 minus the gestational days you enter. Gestational days are weeks times 7 plus days, and days must be 0 through 6.

For last menstrual period, completed weeks are days since that date divided by 7, rounded down, so the period date is week 0. For conception, the count adds 14 days before dividing by 7. For ultrasound, completed weeks start from the gestational age entered and add the days since the scan. A start date still ahead is labeled Before week 0.

Completed weeks 0 through 12 are the first trimester, 13 through 26 the second, and week 27 or later the third. Week 41 and later stay in that third label. These labels follow the week count. They are not a care plan.

280 days is the last-menstrual-period rule and the ultrasound countdown. 266 days is the conception rule, which is 280 minus 14. The result names which rule ran.

No. This page is a date estimate only. It does not provide prenatal care, screening, or a prediction of the birth date.

ACOG Committee Opinion 700 describes the estimated due date as 280 days after the last menstrual period and notes that this count assumes a 28-day cycle. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date

No. This page adds exactly 280 days to the last menstrual period, or 266 days to the conception date. It does not subtract 3 months and add 7 days. Those calendar steps are not the same number of days in every month.

This page uses the 280-day and 266-day rules described in ACOG Committee Opinion 700. Source: ACOG Committee Opinion 700: https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date.

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Due Date

Enter values to calculate

Completed Weeks

Days Remaining

Note:Estimated due date only, not prenatal care. The line under the date names the rule: LMP + 280 days, conception + 266 days, or ultrasound date + (280 − gestational days).