Gestational Age Calculator: Weeks and Due Date

Estimate Pregnancy Age by Last Menstrual Period

Gestational Age Formula

Formula

\[\begin{aligned}L_a &= L+(C-28)\text{ days}\\D &= \text{days from }L_a\text{ to today}\\W &= \left\lfloor D/7\right\rfloor,\qquad R=D-7W\\E &= L_a+280\text{ days}\end{aligned}\]

Where:

\(L\) — first day of the last menstrual period.
\(C\) — usual cycle length in days; 28 is the calculator’s reference length.
\(L_a\) — cycle-adjusted starting date.
\(D\) — elapsed days since \(L_a\); \(W\) and \(R\) are completed weeks and remaining days.
\(E\) — estimated date of delivery.

The calculator shifts the starting date by the difference between your cycle length and 28 days. It then expresses the elapsed time as weeks and days and adds 280 days to the adjusted date for the delivery estimate.

Gestational Age – Calculation Example

Example: The first day of your last menstrual period was July 6, 2026, and your usual cycle length is 28 days. Calculate your gestational age on September 28, 2026.

\[L_a=\text{July 6, 2026}+(28-28)\text{ days}=\text{July 6, 2026}\]

\[D=84\text{ days},\qquad W=84\div7=12,\qquad R=0\text{ days}\]

\[E=\text{July 6, 2026}+280\text{ days}=\text{April 12, 2027}\]

Result: The gestational age is 12 weeks and 0 days on September 28, 2026, and the estimated date of delivery is April 12, 2027.

How to use the calculator

In Last menstrual period, select the first day your most recent menstrual period began—not the day bleeding ended or the date you think conception occurred. The field initially shows today’s date, so change it to your actual date.
In Cycle Length, enter the usual number of days from the first day of one period to the first day of the next. The default is 28 days; the accepted range is 20 to 45 days.
Read Weeks and Days together for your estimated gestational age. Estimated Date of Delivery gives the corresponding due date. The calculation uses today’s date, so the weeks-and-days result changes over time.

What gestational age means

Gestational age describes how far a pregnancy has progressed. It is conventionally counted from the first day of the last menstrual period, rather than from conception. For example, a result of 12 weeks and 0 days means 12 completed weeks have elapsed according to the calculator’s adjusted starting date; it does not mean conception happened 12 weeks ago.

Clinicians use gestational age to place ultrasound findings, fetal growth, screening tests, and pregnancy milestones in context. A 40-week pregnancy is the basis of this calculator’s estimated delivery date, but that date is a reference point, not a prediction of the exact day of birth. If you want to compare methods of estimating that date, see the Pregnancy Due Date calculator.

The cycle-length adjustment accounts for a simple timing assumption: compared with a 28-day cycle, ovulation may occur later in a longer cycle or earlier in a shorter one. A 30-day cycle therefore shifts the calculated starting date two days forward and the delivery estimate two days later. It does not measure when ovulation or conception actually occurred.

Assumptions and limitations

The period date needs to be reliable. If you are unsure when your last period began, or bleeding was not a typical period, the estimate may be inaccurate.
Cycle length is an approximation. Ovulation does not always occur on the same cycle day, even when periods seem regular. Irregular cycles, recent hormonal contraception, breastfeeding, and other factors can make period-based dating less dependable.
Future dates are not meaningful as a current gestational age. Enter a last menstrual period that has already occurred. Near midnight or a daylight-saving-time change, date handling may also produce a one-day difference from a calendar count.
Clinical dating can differ. A healthcare professional may use an early ultrasound and your medical history to establish or revise the pregnancy’s official dating. The Ultrasound Gestation Period calculator and Crown Rump Length calculator can help you understand measurements, but they do not replace a clinical assessment.

Use this result as an estimate for planning and discussion, not as confirmation of pregnancy or a substitute for prenatal care. If your calculated dates and an ultrasound report disagree, ask your healthcare professional which dating estimate to follow.

Frequently Asked Questions

How can I calculate gestational age by hand for a date other than today?

Count the calendar days from your starting date to the date you want to check, then divide by seven. The whole number is completed weeks, and the leftover days are the additional days. If your cycle is not 28 days, first move the starting date forward or backward by the difference. This gives an estimate; an ultrasound may establish different clinical dates.

What gestational age is considered full term?

Full term is generally 39 weeks and 0 days through 40 weeks and 6 days. Birth at 37 weeks and 0 days through 38 weeks and 6 days is called early term; 41 weeks is late term, and 42 weeks or later is postterm. These categories describe timing, not whether an individual pregnancy is healthy. Ask your prenatal care team about your specific situation.

How do I convert gestational weeks into months or trimesters?

Weeks are more precise than months because calendar months have different lengths. A common convention places the first trimester through 13 weeks and 6 days, the second from 14 weeks through 27 weeks and 6 days, and the third from 28 weeks onward. Trimester boundaries can vary between sources, so use weeks and days when discussing appointments or test timing.

Why is gestational age usually greater than the time since conception?

Gestational age starts before conception in the usual dating convention. With a typical 28-day cycle, ovulation and conception may occur roughly two weeks after a period begins, so the time since conception is often about two weeks less than gestational age. That gap is not fixed: ovulation timing varies, and a period-based estimate cannot determine the exact day conception occurred.

Can I use a last-period calculator to date a pregnancy after IVF?

IVF pregnancies are usually dated from the embryo transfer date and the embryo’s age at transfer, rather than from a menstrual period. Entering a period date here may produce a different result from the dates supplied by your fertility clinic. Use the clinic’s established gestational age and due date for prenatal planning, and ask your care team if the dates appear inconsistent.

What if my estimated due date has already passed?

A due date passing does not, by itself, mean something is wrong; it is an estimate rather than a deadline. Gestational age continues to increase beyond 40 weeks, and your care team may recommend additional monitoring or discuss delivery timing based on your circumstances. Contact them for advice rather than using the calculator alone to decide what to do next.

Why might the calculator show zero or negative weeks just after my period?

A long cycle can shift the calculator’s starting date forward to a day that has not yet arrived, producing a zero or negative result. A future or incorrectly entered period date can do the same. Check the date and cycle length first. If the entries are correct, the result is a limitation of this simple cycle adjustment—not a meaningful clinical gestational age.

Gestational Age Calculator