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.
Dana checks her dates before booking a flight
Dana wants to know how far along she will be on October 7 because her airline asks for gestational age on a fitness-to-fly form. From April 20 to October 7 is 170 days. Dividing by 7 gives 24 completed weeks because 24 × 7 = 168, with 2 days left over, so the tool reports 24 weeks, 2 days. That is still second trimester under the tool's 28-week cutoff, and 170 ÷ 280 = 60.7% of the standard timeline. The estimated due date is April 20 plus 280 days, which lands on January 25, 2027. Dana's 12-week scan matched her LMP within 2 days, so under ACOG Committee Opinion 700 her LMP-based date stands and the number on this page is the one her clinic uses too.
Gestational milestones for an LMP of January 1, 2026
Each row shows the calendar date on which the tool reaches that milestone for an LMP of January 1, 2026, along with the trimester and percent-complete figures it returns on that date. Add the same day count to any other LMP to find your own dates.
| Milestone | Days from LMP | Date reached | Tool trimester | Percent complete | Clinical label |
|---|---|---|---|---|---|
| 6 weeks, 0 days | 42 | Feb 12, 2026 | First | 15% | Embryonic period, crown-rump dating becomes possible |
| 12 weeks, 0 days | 84 | Mar 26, 2026 | First | 30% | Inside the first-trimester window, through 13 weeks 6 days, that ACOG 700 calls most accurate for ultrasound dating |
| 14 weeks, 0 days | 98 | Apr 9, 2026 | Second | 35% | Tool switches to second trimester |
| 20 weeks, 0 days | 140 | May 21, 2026 | Second | 50% | Halfway through the 280-day timeline |
| 24 weeks, 0 days | 168 | Jun 18, 2026 | Second | 60% | Common threshold in periviability guidance |
| 28 weeks, 0 days | 196 | Jul 16, 2026 | Third | 70% | Tool switches to third trimester. WHO: extremely preterm ends |
| 32 weeks, 0 days | 224 | Aug 13, 2026 | Third | 80% | WHO: very preterm ends, moderate to late preterm begins |
| 37 weeks, 0 days | 259 | Sep 17, 2026 | Third | 92.5% | WHO preterm ends. ACOG 579: early term begins |
| 39 weeks, 0 days | 273 | Oct 1, 2026 | Third | 97.5% | ACOG 579: full term begins |
| 40 weeks, 0 days | 280 | Oct 8, 2026 | Third | 100% | Estimated due date, LMP plus 280 days |
| 41 weeks, 0 days | 287 | Oct 15, 2026 | Third | 100% (capped) | ACOG 579: late term begins |
| 42 weeks, 0 days | 294 | Oct 22, 2026 | Third | 100% (capped) | ACOG 579: postterm begins |
Dates, trimester and percent columns computed with the formula above (day count ÷ 7, trimester cutoffs at 14 and 28 completed weeks, percent = days ÷ 280 capped at 100). Clinical labels follow the WHO preterm birth definitions and ACOG Committee Opinion 579 (2013) on the definition of term pregnancy. They describe the week ranges only and are not predictions about any individual pregnancy.
How clinics date a pregnancy and when the LMP count gets replaced
Counting from the last menstrual period is the oldest dating method in obstetrics. Naegele's rule, published in the early 1800s, sets the due date by adding one year, subtracting three months and adding seven days to the LMP, which is the same as adding 280 days in most months. That convention assumes a 28-day cycle with ovulation on day 14, so gestational age runs about two weeks ahead of the time since conception. This calculator applies exactly that convention. It counts calendar days from the LMP to the reference date, splits them into completed weeks and leftover days, and adds 280 days to the LMP for the estimated due date.
The American College of Obstetricians and Gynecologists, in Committee Opinion 700 (2017), written with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine, treats an early ultrasound as the most accurate way to set or confirm gestational age. The opinion says the due date should be set as soon as LMP data, a first accurate ultrasound, or both are available, documented clearly, and then changed only in rare circumstances. It also says a pregnancy without an ultrasound that confirms or revises the due date before 22 weeks and 0 days should be considered suboptimally dated. In practice, when a first-trimester crown-rump measurement disagrees with the LMP date by more than a few days, the ultrasound date wins, and the allowable gap grows with gestational age because measurement error grows too. The exact thresholds are tabulated in that opinion, and clinicians apply them rather than this page.
The trimester and term labels on this page come from two different places. The trimester cutoffs at 14 and 28 completed weeks are the tool's own convention, chosen because they are common in patient materials, and other sources draw the first-to-second boundary at 12 or 13 weeks. The term labels in the reference table follow ACOG Committee Opinion 579 (2013), which split the old single term window into early term (37 weeks 0 days through 38 weeks 6 days), full term (39 weeks 0 days through 40 weeks 6 days), late term (41 weeks 0 days through 41 weeks 6 days) and postterm (42 weeks 0 days and beyond). The World Health Organization defines preterm birth as birth before 37 completed weeks and subdivides it into extremely preterm (under 28 weeks), very preterm (28 to under 32 weeks) and moderate to late preterm (32 to under 37 weeks). Those labels classify births by week. They say nothing about how any individual pregnancy is progressing.
The method has clear limits. It assumes the LMP date is remembered accurately and that ovulation happened around day 14, which fails with irregular cycles, recent hormonal contraception, breastfeeding, or bleeding that was not a true period. Pregnancies from IVF are dated from the transfer date and embryo age under Committee Opinion 700, not from an LMP, so this page does not apply to them. The percent-complete figure is a share of a 280-day calendar, not a measure of fetal development, and it stops at 100 even though many pregnancies continue past 40 weeks. Treat the output as a quick weeks-and-days translation of two dates, and use the dating your clinician has documented for anything that affects 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, the convention behind Naegele's rule. It does not account for irregular cycles, uncertain recall of the LMP date, implantation timing, or ultrasound-based adjustments. ACOG Committee Opinion 700 (2017) treats an early ultrasound as the most accurate dating method, replaces the LMP date when the two disagree by more than its tabulated thresholds, dates IVF pregnancies from the transfer date rather than an LMP, and considers a pregnancy without a confirming ultrasound before 22 weeks 0 days suboptimally dated. The trimester cutoffs at 14 and 28 weeks are the tool's own convention, while the term and preterm labels in the reference table follow ACOG Committee Opinion 579 (2013) and the WHO preterm definitions and describe week ranges only. 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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