Gestational Age Calculator
An LMP of January 1, 2026, puts a pregnancy at 27 weeks, 4 days on July 13, 2026. This gestational age calculator estimates how far along a pregnancy is on a chosen date by counting from the first day of the last menstrual period, often shortened to LMP. Enter the LMP date and a reference date, and the tool returns the current gestational age in completed weeks and extra days, the trimester, the total number of days counted so far, the percent of a standard 280-day pregnancy completed, and the estimated due date. This page focuses on age-so-far rather than prediction alone. That matters because many people do not just want a due date. They want to know what 9 weeks, 3 days or 31 weeks, 0 days means on a visit date, scan date, travel date, or insurance form. The calculator uses the same simple LMP-based convention used in many prenatal settings, where pregnancy dating starts before conception because the menstrual cycle provides a clear date to count from. It is best used for a quick estimate, a scheduling check, or a plain-language explanation of gestational weeks. It is not a substitute for ultrasound dating, prenatal care, or individualized advice from a clinician, especially when the LMP date is uncertain or cycles are irregular.
Quick answer
Gestational age counts from the first day of the last menstrual period, not from the day of conception.
What this tells you
- •Gestational age counts from the first day of the last menstrual period, not from the day of conception.
- •The result is shown as completed weeks plus remaining days because that is the format commonly used in prenatal care notes.
- •The calculator counts the exact day difference between the LMP date and the reference date, then divides by 7 to separate weeks from extra days.
- •Trimester labels in this tool follow a simple week-based split. Weeks 0 through 13 are first trimester, weeks 14 through 27 are second trimester, and week 28 onward is third trimester.
- •Percent complete is based on a standard 280-day pregnancy, which equals 40 weeks from LMP.
- •The estimated due date is the LMP date plus 280 days, even if the current reference date is already past that mark.
How to Use
- 1Enter the first day of the last menstrual period as accurately as possible. If you are unsure of the exact date, treat the result as a rough estimate rather than a firm dating answer.
- 2Enter the reference date you want to measure against. In most cases this is today, but you can also check a past appointment date or an upcoming travel date.
- 3Run the calculator to see the current gestational age in weeks and days. For example, a result of 18 weeks, 2 days means 18 full weeks have passed and the pregnancy is 2 days into week 19.
- 4Review the trimester, percent through pregnancy, and estimated due date for extra context. These fields help you place the week count into a broader timeline without doing any additional calendar math.
- 5If your provider has dated the pregnancy by ultrasound or has told you to use a different due date, treat that clinical dating as the more reliable reference for care decisions.
How It Works
Formula
Gestational Age = Reference Date - LMP DateThe formula in code is a day-count method. First, the calculator reads the LMP date and the reference date in YYYY-MM-DD format and converts each one to a UTC calendar date. Next, it subtracts the LMP date from the reference date to get the total number of days pregnant so far. If that number is negative, the result is invalid because the reference date cannot come before the LMP date. Once the day count is known, the calculator divides by 7. The whole-number part becomes completed gestational weeks, and the remainder becomes extra days. For example, 193 total days becomes 27 completed weeks because 27 × 7 = 189, with 4 days left over, so the result is 27 weeks, 4 days. The estimated due date is calculated separately as LMP date + 280 days. Percent complete is total days pregnant ÷ 280 × 100, rounded to one decimal place and capped at 100. Trimester is assigned from completed weeks only, with cutoffs at 14 and 28 weeks.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Second trimester visit date
From January 1 to July 13 is 193 days. Dividing 193 by 7 gives 27 full weeks with 4 days left over, so the gestational age is 27 weeks, 4 days. The tool marks this as second trimester because completed weeks are still below 28, and 193 ÷ 280 = 68.9% when rounded to one decimal place.
Early first trimester check
The date difference is 35 days. Since 35 ÷ 7 = 5 exactly, there are no extra days, so the result is 5 weeks, 0 days. The percent complete is 35 ÷ 280 = 12.5%, which makes this a straightforward example of an early dating check.
Mid-pregnancy anatomy scan timing
From February 14 to August 1 is 168 days. Dividing 168 by 7 gives 24 full weeks with no remainder, so the pregnancy is exactly 24 weeks along on that reference date. The percent complete is 168 ÷ 280 = 60%, which shows the pregnancy is a little past the halfway point of the standard 40-week timeline.
Late second trimester planning check
This span covers 190 total days. When you divide 190 by 7, you get 27 completed weeks because 27 × 7 = 189, with 1 day remaining. The percent complete rounds to 67.9%, which is useful when someone wants both a week count and a sense of where they sit in the overall pregnancy timeline.
Third trimester milestone check
The day difference here is 259 days. Dividing by 7 gives 37 full weeks and 0 extra days, so the result lands squarely in the third trimester. Since 259 ÷ 280 = 92.5%, this example shows how the tool behaves when the pregnancy is close to the estimated due date but not yet at 40 weeks.
How to read a gestational age result
A quick guide to the terms this calculator returns.
| Result item | What it means | How the tool sets it |
|---|---|---|
| Weeks and days | Completed weeks plus extra days into the next week | Total day difference split by 7 |
| Trimester | Broad stage of pregnancy | First: under 14 weeks, Second: 14 to 27 weeks, Third: 28 weeks and up |
| Percent complete | Share of a standard 280-day pregnancy | Total days divided by 280 and rounded to one decimal place |
| Estimated due date | Standard 40-week due date from LMP | LMP date plus 280 days |
Different clinics may discuss trimester boundaries slightly differently. This tool follows the week cutoffs used in its formula code.
Why LMP dating can differ from conception timing
Gestational age is intentionally counted from the first day of the last menstrual period because that date is often easier to identify than the exact day of ovulation or conception. In a typical cycle, ovulation happens about two weeks after the LMP. That means gestational age is usually about two weeks higher than conception-based age.
This difference explains a common point of confusion. Someone may know the likely week of conception and wonder why the pregnancy seems further along than expected. The answer is that obstetric dating starts earlier on purpose. It gives clinicians and patients one shared clock to use for visit timing, screening windows, and chart notes.
LMP dating works best when menstrual cycles are fairly regular and the LMP date is remembered clearly. If cycles vary a lot, if bleeding was unusual, or if the LMP date is uncertain, the simple date-count method can drift away from the best clinical estimate. In those cases, ultrasound dating may lead to a different gestational age or due date.
That is why this calculator is most useful for estimation and education. It translates two dates into the common weeks-and-days format quickly, but it does not try to replace the dating method a healthcare professional has chosen for actual prenatal care.
Common mistakes
- Entering the conception date instead of the first day of the last menstrual period. Gestational age almost always starts earlier than conception-based timing.
- Using an estimated LMP date from memory when the exact date is uncertain, then treating the result as exact.
- Assuming the due date tells you the same thing as the current gestational age. One projects forward, while the other describes where the pregnancy stands on a specific date.
- Reading 18 weeks, 6 days as if it were nearly 19 completed weeks. The weeks value is the completed part, and the days value shows progress into the next week.
- Treating trimester labels as precise medical boundaries for every decision. Real care recommendations depend on more than one simple week cutoff.
- Ignoring provider-adjusted dating after an ultrasound. If a clinician has updated the due date, that revised dating usually matters more than a basic LMP estimate.
Limitations
This estimate assumes a standard 280-day, 40-week pregnancy dated from the first day of the last menstrual period. It does not account for irregular cycles, uncertain recall of the LMP date, implantation timing, fertility treatment dating, ultrasound-based adjustments, multiple gestation, or provider-specific documentation rules. The trimester label is based only on completed weeks, and the percent-complete figure is a timeline aid rather than a statement about fetal development or delivery timing. A real pregnancy can measure ahead of or behind this estimate without anything being wrong, which is why clinical dating may differ from a simple calendar count.
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