Gestational Age Calculator

The gestational age calculator estimates pregnancy age in weeks and days from last menstrual period, conception date, or ultrasound dating. It also shows an estimated due date, estimated conception date, trimester, developmental milestone dates, and suggested prenatal appointment timing. Because gestational age is clinical information, use the result as an educational estimate and confirm it with your healthcare professional.

Before you use the result

Pregnancy dating affects screening windows, ultrasound interpretation, medication timing, and conversations about preterm or post-term pregnancy. A calculator can reproduce standard date arithmetic, but it cannot decide which clinical date should be authoritative. ACOG emphasizes using the best obstetric estimate, and early ultrasound can be important when menstrual dates are uncertain. If you have pain, bleeding, uncertain dates, assisted reproduction timing, irregular cycles, or conflicting scan results, do not rely on a web calculator for care decisions.

For related planning, compare a conception-focused estimate with the pregnancy conception calculator, work backward from an expected delivery date with the reverse due date calculator, or use the pregnancy due date calculator for a due-date-centered view. For nonpregnancy birthdays and eligibility dates, use the age calculator.

Method and calendar rules

The calculator has three calculation paths. In the last menstrual period path, startDate is the LMP date and gestational days are the calendar days from that date to the calculate-as-of date:

$\text{gestational days} = \left\lfloor \operatorname{daysBetween}(\text{LMP date},\ \text{as-of date}) \right\rfloor$

In the conception date path, the calculation subtracts 14 days from the entered conception date to create a gestational dating start. It then counts days from that start date to the as-of date:

$\text{dating start} = \text{conception date} - 14\ \text{days}$

$\text{gestational days} = \left\lfloor \operatorname{daysBetween}(\text{dating start},\ \text{as-of date}) \right\rfloor$

In the ultrasound path, the scan age is converted to days and the elapsed days since the scan are added:

$\text{gestational days} = 7 \times \text{ultrasound weeks} + \text{ultrasound days} + \left\lfloor \operatorname{daysBetween}(\text{ultrasound date},\ \text{as-of date}) \right\rfloor$

The calculator then sets the start date by subtracting gestational days from the as-of date. It rejects results below 0 days or above 294 days, which is 42 weeks. Current week is whole gestational days divided by 7, and current day is the remainder:

$\text{current week} = \left\lfloor \frac{\text{gestational days}}{7} \right\rfloor$

$\text{current day} = \text{gestational days} \bmod 7$

The due date is 280 days after the dating start date, and the estimated conception date is 14 days after the dating start date. Trimester is 1 through week 13, 2 through week 26, and 3 starting week 27.

Example: estimating gestational age

Choose Last Menstrual Period, enter 2026-01-01 as the LMP date, and set Calculate as of to 2026-03-05. The date difference from January 1 to March 5 is 63 calendar days. Dividing by 7 gives 9 weeks with 0 days remaining, so the primary result is 9 weeks, 0 days. Since week 9 is less than or equal to 13, the hint shows Trimester 1.

The dating start is January 1. The due date is 280 days later, October 8, 2026. The estimated conception date is 14 days after the dating start, January 15, 2026. Milestone and appointment rows are produced by adding each listed week number times 7 days to January 1. For example, the week 12 milestone is dated 84 days after January 1.

For an ultrasound example, suppose a scan on 2026-03-01 reports 8 weeks, 3 days, and the as-of date is 2026-03-08. The scan age is 59 days. Seven more days have elapsed, so gestational days are 66, or 9 weeks, 3 days.

Interpreting milestones and appointments

The milestone list is a general timeline generated from the dating start. It includes items such as heartbeat may be detected, anatomy scan ultrasound, third trimester begins, early term begins, and due date. The appointment list similarly places common prenatal events at week numbers. Those labels are not instructions. Your clinician may use different timing depending on risk factors, local practice, screening choices, prior pregnancy history, or scan findings.

Gestational age is conventionally expressed as completed weeks plus days, such as 9 weeks, 0 days or 20 weeks, 4 days. Someone at 9 weeks, 6 days is still in week 9 by this calculator’s display, though conversational language may say “in the tenth week.” Use the exact weeks-and-days format when discussing tests or appointments.

Edge cases and common mistakes

Irregular cycles can make LMP dating less reliable. The conception-date path assumes a 14-day difference between conception and gestational dating start, which is a convention rather than proof of ovulation. Ultrasound dating depends on scan timing and measurement quality; early scans are generally more useful for dating than later growth scans. The calculator also requires the dating input to be on or before the as-of date and rejects gestational age above 42 weeks.

Leap years are handled by calendar-day arithmetic in the date library. Daylight saving time does not matter for date-only inputs. The most serious mistake is treating the estimated conception date as evidence about intercourse timing or paternity. It is not. It is a date estimate produced from pregnancy dating assumptions.

Calendar and date boundaries

Inputs are Gregorian date-only values. Month lengths and February 29 are preserved when calendar days are added or subtracted. Time zones and daylight-saving transitions are not inputs and do not add or remove a gestational day; the estimate changes when the as-of calendar date changes. A dating start after the as-of date is outside the calculator’s domain.

Sources

• ACOG, Methods for Estimating the Due Date — clinical guidance on pregnancy dating and the best obstetric estimate.
• MedlinePlus, Gestational age — patient-facing definition of gestational age.
• ACOG, How Your Fetus Grows During Pregnancy — general pregnancy development timeline for patient education.