Gestational Age Calculator

A gestational age calculator, sometimes called a pregnancy week calculator, converts a date into the weeks-and-days figure your provider writes in your chart. Count from the first day of your last period: an LMP of January 5, 2026 means that on April 24, 2026 you are 15 weeks 4 days pregnant, or 109 days, in the second trimester, with an estimated due date of October 12, 2026 and 171 days to go.

This page and the Pregnancy Due Date Calculator are siblings that answer opposite questions. Use the due date calculator when you want to know when the baby arrives, including from an IVF transfer date. Use this page when you want to know how far along you are right now, or on any other date: a scan appointment, a travel cut-off, the first day of maternity leave. This calculator also works backwards from a due date your provider already gave you, which the due-date tool does not do.

What gestational age actually means

Gestational age is the age of a pregnancy counted from the first day of the last menstrual period (LMP), not from conception. That convention exists for a practical reason: almost nobody knows the hour an egg was fertilized, but most people can name the day their last period started. Because ovulation happens roughly 14 days into a typical 28-day cycle, gestational age runs about two weeks ahead of the true age of the embryo. ACOG puts it plainly: pregnancy is assumed to start 2 weeks after the first day of the last period. That is why you are already 4 weeks pregnant on the day your next period would have been due, and why the pregnancy is counted as two weeks old on the day conception actually happens.

The unit is always completed weeks plus extra days, written as 15w 4d or "15 and 4". Nobody rounds up. At 15 weeks 6 days you are still 15 weeks, and you become 16 weeks the next morning. That precision matters because screening windows, medication decisions and growth references are all keyed to specific week-and-day boundaries.

How gestational age is calculated

Every mode in this calculator reduces to the same subtraction. In days:

and the weeks-and-days form is simply that number split by 7:

When you do not have the LMP, the other two modes convert to it first:

• From a due date: a full-term pregnancy is 280 days, so GA = 280 − days until the due date. Equivalently, the implied LMP is the due date minus 280 days.
• From conception or ovulation: GA = days since conception + 14, because the count starts about two weeks before conception. The due date is conception plus 266 days.
• From an LMP: the estimated due date follows Naegele's rule, EDD = LMP + 280 days, optionally shifted by your cycle length minus 28. With a shift, the weeks are counted from the matching adjusted start, so the due date is always 40w 0d.

Worked example: counting from a last period

Suppose your last period started on January 5, 2026 and you want to know how far along you are on April 24, 2026.

1. Days elapsed: January 5 to April 24, 2026 is 109 days.
2. Whole weeks: 109 ÷ 7 = 15 remainder 4, so gestational age is 15 weeks 4 days.
3. Trimester: 15w 4d is past the end of the first trimester (13w 6d) and before the third begins (28w 0d), so you are in the second trimester.
4. Due date: January 5 + 280 days = October 12, 2026.
5. Days remaining: April 24 to October 12 is 171 days, and 109 ÷ 280 = 38.9% of the 40 weeks completed.
6. Other units: 109 ÷ 7 = 15.6 weeks, and 109 ÷ 30.44 = 3.6 months.

Conception in this example landed near January 19, 2026, two weeks after the LMP, so the embryo itself is about 13 weeks 4 days old even though the pregnancy is dated 15 weeks 4 days.

Worked example: counting back from a known due date

If a first-trimester scan already set your due date, that date beats any calculation from memory. Say your provider gave you November 15, 2026 and you want your gestational age on June 1, 2026. There are 167 days between the two dates, so the gestational age is 280 − 167 = 113 days, which is 16 weeks 1 day, the second trimester, and 40.4% of the way through. The implied first day of the last period is November 15 minus 280 days, which is February 8, 2026. If that implied date is more than a week away from the period you actually remember, your dates were probably revised by ultrasound, and the scan date is the one to keep using.

Worked example: counting from a conception date

People using ovulation tracking or an IUI often know the conception day precisely; after IVF, the fertilization (egg retrieval) date plays the same role. Take March 2, 2026 as the conception date and July 10, 2026 as the date of interest. Days since conception: 130. Add the standard 14-day offset and gestational age is 144 days, which is 20 weeks 4 days. The implied LMP is March 2 minus 14 days, or February 16, 2026, and the due date is March 2 plus 266 days, November 23, 2026, with 136 days left to go. Both routes agree, because 266 days after conception is the same calendar day as 280 days after the implied last period.

Milestone dates for a January 5, 2026 last period

Every date below is that LMP plus the listed number of gestational weeks, using the same arithmetic the calculator runs on your own dates.

How far along on a given date

The same LMP of January 5, 2026, read at eight different dates. This is the table people are really looking for when they search "how many weeks pregnant am I": pick the row nearest your date, or enter your own dates above for the exact figure.

Days until the due date, converted to gestational age

If the only date you have is your due date, this conversion works for any pregnancy, because it depends on nothing but the 280-day span. Subtract the days remaining from 280 and split the result by 7.

The three trimesters

Trimesters are a convenient thirds-of-pregnancy split rather than a biological boundary. This calculator uses the definitions of the American College of Obstetricians and Gynecologists (ACOG):

• First trimester, first day of the LMP to 13w 6d. ACOG describes it as the time when fertilization and major organ development occur. An ultrasound in this window is the most accurate way to date the pregnancy (ACOG Committee Opinion 700).
• Second trimester, 14w 0d to 27w 6d. A time of rapid growth and development. The standard anatomy ultrasound is usually done at 18 to 22 weeks.
• Third trimester, 28w 0d to birth. The fetus gains weight and the organs mature for life after birth.

Because the boundaries land on exact week-and-day marks, the difference between "end of the first trimester" and "start of the second" is a single day. This calculator starts the second trimester at 14w 0d and the third at 28w 0d.

Preterm, early term, full term, late term and post-term

"Term" is not one block. Since ACOG and the Society for Maternal-Fetal Medicine adopted these designations (Committee Opinion 579), a birth at 37 weeks and a birth at 40 weeks are described differently, because those weeks matter for newborn outcomes:

• Preterm: before 37w 0d.
• Early term: 37w 0d through 38w 6d.
• Full term: 39w 0d through 40w 6d.
• Late term: 41w 0d through 41w 6d.
• Post-term: 42w 0d and beyond.

The calculator labels which band your entered date falls into, which is useful when you are weighing something like an elective scheduling decision against the calendar. Discuss anything clinical with your provider, who has your full picture.

How to use this calculator

1. Pick what you know. Last period, a due date your provider gave you, or a conception or ovulation date. Any one of the three is enough.
2. Enter that date. For the LMP, use the first day of bleeding, not the last day.
3. Set the "calculate as of" date. It defaults to today. Change it to a scan date, a flight date, or the day you plan to start leave to see the gestational age you will be then.
4. Choose a display unit. Weeks + days is what clinicians use; decimal weeks and months are there for everyday conversation.
5. Optionally adjust cycle length. In LMP mode this shifts the estimated due date by your cycle length minus 28 days and counts the weeks from the matching adjusted start.
6. Read the result. The headline shows your gestational age, and below it you get the trimester, term status, due date, days to go, the next few week markers and a full milestone timeline.

Who this calculator is for

• Anyone who just found out. The first question after a positive test is usually "how far along am I", and the answer is counted from the last period, not from conception.
• People tracking a specific week. Screening windows and the standard anatomy scan (usually 18 to 22 weeks) are tied to week ranges, so knowing your exact weeks-and-days keeps appointments on schedule.
• Anyone planning around dates. Travel policies, work leave and event calendars all need "what week will I be on this date", which is exactly what the as-of field answers.
• Partners and family who want to follow along without doing calendar arithmetic in their head.
• Anyone whose dates were revised by an ultrasound and who now needs to work forward from the new due date instead of the remembered period.

Weeks, months and why 40 weeks is not 10 months

A 40-week pregnancy is 280 days, and 280 ÷ 30.44 (the average calendar month) is about 9.2 months, not 10. The confusion comes from treating a month as exactly four weeks: 40 weeks ÷ 4 = 10, but real months average about 4.35 weeks. That is why week-based and month-based counts drift apart as pregnancy goes on. At 20 weeks the two systems are about half a month apart; by 40 weeks the gap is close to a full month. Clinical care always uses weeks and days for this reason, and month talk is best kept for casual conversation. If you want the month figure anyway, the calculator shows it as a display option.

Key terms

• LMP: last menstrual period, specifically the first day of it. The zero point for all gestational dating.
• EDD: estimated due date, the day you reach 40 weeks 0 days.
• Gestational age: weeks and days since the LMP.
• Fetal age: also called conceptional age, counted from conception. Roughly 14 days less than gestational age.
• Naegele's rule: the classic due-date rule, LMP plus 280 days. The calendar shortcut (LMP plus one year, minus three months, plus seven days) lands 0 to 3 days after the 280-day date, depending on the months it spans.
• CRL: crown-rump length, the embryo measurement a first-trimester ultrasound uses to estimate gestational age.
• Re-dating: replacing an LMP-based due date with an ultrasound-based one when the two disagree by more than a set number of days (ACOG: more than 5 days before 9 weeks, more than 7 days from 9 weeks to 15 weeks 6 days).

What changes the result

• The LMP you enter. Every day of error in the last-period date moves the gestational age and the due date by the same day. This is the single biggest source of variation.
• Cycle length. A 35-day cycle usually means ovulation around day 21 rather than day 14, so a pregnancy dated from the LMP is really about a week younger than the raw count.
• Ultrasound re-dating. A first-trimester scan can move the due date by a week or more, and once it does, that scan date becomes the reference for everything afterwards.
• Which date you are asking about. Gestational age is a moving target, which is why the as-of field exists. Every seven days you gain one week.
• Assisted conception. IVF dating is unusually precise because the fertilization and transfer days are recorded, so those dates should override any period-based estimate.

Tips for keeping your dates straight

• Write the LMP down the moment you get a positive test, before the date blurs.
• If you track cycles in an app, screenshot the last period start rather than relying on memory at your first appointment.
• After a dating ultrasound, switch this calculator to due-date mode and use the scan-based date from then on.
• Keep one source of truth. Mixing an old LMP estimate with a revised due date is how people end up two weeks off.
• When someone asks "how many months", answer in weeks first and convert afterwards. Weeks are unambiguous, months are not.
• Check the as-of date if a result looks surprising: it is easy to leave a future date in the field from a previous look.

Limitations and assumptions

• It assumes a 280-day pregnancy from the LMP. Individual pregnancies vary; ACOG notes that only about 1 in 20 women give birth on their due date.
• It assumes ovulation near day 14 of your cycle unless you change the cycle length. Irregular cycles make LMP-based dating unreliable.
• It does not incorporate ultrasound measurements, which are the accurate way to date a pregnancy when the LMP is uncertain.
• It does not adjust for twins or higher-order multiples, which are managed on a different timeline.
• It says nothing about fetal growth, health or risk. It is calendar arithmetic, not a clinical assessment.
• Results are an estimate for information only and are not medical advice.

How it compares to related calculators

This page answers "how far along am I on this date". If your question is a different shape, a sibling tool fits better:

• To estimate when the baby is due from a last period, a conception date or an IVF transfer, use the Pregnancy Due Date Calculator.
• To work backwards from a due date to the likely conception date and fertile window, use the Reverse Due Date Calculator.
• To count days or weeks between any two calendar dates, including appointment intervals, use the Date Calculator.
• To work out an exact age in years, months and days once the baby arrives, use the Age Calculator.
• For a pre-pregnancy weight baseline to discuss with your provider, use the BMI Calculator.

Sources

• American College of Obstetricians and Gynecologists (ACOG): Committee Opinion 700, Methods for Estimating the Due Date (reaffirmed 2025: LMP + 280 days, first-trimester ultrasound up to 13 weeks 6 days, redating thresholds).
• ACOG: Committee Opinion 579, Definition of Term Pregnancy (early, full, late and post-term).
• ACOG: How Your Fetus Grows During Pregnancy (trimester definitions; about 1 in 20 women give birth on the due date) and Ultrasound Exams (standard exam usually at 18 to 22 weeks).
• NIH Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD): Pregnancy: condition information.
• NICHD: Preterm labor and birth.
• NIH National Library of Medicine, MedlinePlus: Prenatal care.