Pregnancy estimate

Pregnancy due date calculator

Estimate the due date, gestational age, trimester and reference milestones from a known date. This result is informational and does not replace professional prenatal care.

Medical information, not a diagnosis

This calculator cannot confirm a pregnancy, assess fetal development or determine when labour will begin. First-trimester ultrasound is generally the most accurate method to establish or confirm gestational age. Contact a midwife, obstetric clinician or other qualified professional for personal care.

Enter pregnancy dates

Use the first day of the last menstrual period when known. A conception date can be used as an alternative estimate.

Calculation method

First day of last periodEstimated conception date

First day of last period

Use the first day of bleeding, not the last day of the period.

Usual cycle length

Enter a whole number from 20 to 45 days. The standard calculation uses 28 days.

Date used for gestational age

This is normally today, but a past reference date may be selected.

Your dates remain on this device

The entered dates and cycle length are calculated locally in your browser and are not sent to a server or stored in the page URL.

How the estimate is calculated

Pregnancy calculation convention counts gestational age from the first day of the last menstrual period, approximately two weeks before conception in the standard 28-day-cycle model.

From the last menstrual period

EDD = LMP + 280 days + (cycle length - 28 days)

The standard estimate is 40 weeks from the first day of the last period, adjusted by the difference between the entered cycle length and 28 days.

From an estimated conception date

EDD = conception date + 266 days

This is the analogous 38-week estimate from conception. Gestational age is still displayed using the conventional two-week offset.

Why cycle length changes the estimate

The standard rule assumes a 28-day cycle with ovulation near day 14. A longer or shorter regular cycle shifts the estimated conception and due dates by the same number of days. Irregular cycles make this adjustment less reliable.

Trimester convention used

The page follows the week-and-day boundaries used by ACOG for general pregnancy descriptions.

First trimester

0 weeks 0 days to 13 weeks 6 days

From the first day of the last period through gestational day 97.

Second trimester

14 weeks 0 days to 27 weeks 6 days

Gestational days 98 through 195.

Third trimester

28 weeks 0 days onward

From gestational day 196 until delivery.

Important limitations

The estimated due date is not a prediction of the actual day of birth.
Irregular cycles, uncertain dates, recent hormonal contraception, breastfeeding and assisted reproduction may require a different calculation method.
Ultrasound measurements and clinical information can lead a clinician to select a different official due date.
The 37-to-42-week dates are a reference interval, not a promise that birth will occur inside it.
Urgent symptoms or concerns require medical assessment, not an online calculator.

Examples

Standard 28-day cycle

LMP 1 January 2026 + 280 days

Estimated due date: 8 October 2026

30-day cycle

LMP 1 January 2026 + 282 days

Estimated due date: 10 October 2026

Known conception estimate

15 January 2026 + 266 days

Estimated due date: 8 October 2026

Frequently asked questions

Is the estimated due date exact?

No. It is a reference date for 40 completed weeks. Birth commonly occurs before or after that date.

Which date should I use if I am unsure of my last period?

Discuss the uncertainty with a qualified prenatal professional. Early ultrasound is generally more accurate than guessing an uncertain last-period date.

Why does pregnancy start before conception in the result?

Clinical gestational age is conventionally counted from the first day of the last menstrual period, typically about two weeks before conception in the standard model.

Can this calculator replace an ultrasound date?

No. The calculator is informational. A clinician uses ultrasound findings and clinical history to establish the official due date.

Sources

ACOG - Methods for Estimating the Due Date
Clinical recommendations on establishing gestational age and the estimated due date, including the role of first-trimester ultrasound.
NHS - Due date calculator
Public guidance using the first day of the last period and cycle-length adjustment.
ACOG - How Your Fetus Grows During Pregnancy
Definitions of pregnancy weeks and trimester boundaries.