Gestational Age Calculator · Pregnancy Weeks & Days from LMP, Ultrasound or IVF

Gestational Age Calculator

Calculate your pregnancy weeks and days from LMP, ultrasound, IVF transfer, or conception date

Dating method

How would you like to calculate?

First day of last menstrual period

Average cycle length

days

Naegele's rule: EDD = LMP + 280 days, adjusted for cycles shorter or longer than 28 days.

Ultrasound date

Gestational age at scan

weeks days

First-trimester ultrasound is the most accurate dating method — within ±3–5 days before 14 weeks.

Embryo transfer date

Embryo stage at transfer

IVF dating is the most precise — GA = transfer date + (14 + embryo age) days.

Conception date

Use only if the exact day of ovulation or fertilisation is known (e.g. single intercourse or IUI).

Gestational age is counted from the first day of your last menstrual period — about 2 weeks before conception. This is the universal standard used by obstetricians and neonatologists worldwide.

Quick examples

Your Gestational Age

Gestational age today

—

— due date — conception —

Gestational age

—

Days remaining

— days

Weeks remaining

— weeks

Trimester

—

Pregnancy progress

1st trimester 2nd trimester 3rd trimester

Your pregnancy milestones

Milestone	Gestational age	Date	Notes

Trimester breakdown

Trimester	Weeks	Date range	Key developments

Term reference

Category	Gestational age	Date range	Notes

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Creator & Maintainer

Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.

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Gestational age isn’t the same as fetal age. It’s measured from the first day of your last menstrual period, roughly two weeks before ovulation and fertilization happened. That two-week offset is baked into every prenatal chart, every scan report, and every conversation about “how far along” you are. It’s also why a 40-week pregnancy only represents about 38 weeks of actual fetal development.

Four methods produce this number, and they don’t always agree. LMP dating assumes a 28-day cycle with ovulation on day 14. Ultrasound dating measures the embryo directly. IVF dating uses the embryologist’s exact record of embryo age. Conception dating works only if you know the day of fertilization with certainty, which is rare outside of IUI or a single timed intercourse window.

How to Use the Gestational Age Calculator

Choose a dating method from the dropdown: LMP, ultrasound, IVF transfer, or conception date.
Fill in the date the method asks for. The form swaps to match your choice.
Adjust cycle length for LMP if yours isn’t 28 days.
Set weeks and days separately for an ultrasound. Both fields accept whole numbers.
Pick the embryo stage (day 3, 5, or 6) for IVF.
Read the hero panel. Your gestational age today appears as “X weeks Y days” in large type.
Check the four tiles for gestational age, days remaining, weeks remaining, and trimester.
Scroll the milestone table for dated events from conception through your estimated due date.

How the Gestational Age Calculator Formula Works

Formula: Gestational age = days since LMP, divided into weeks and days

Every method anchors to the LMP date, then counts forward to today.

For LMP dating, the due date is LMP + 280 days adjusted for cycle length:

Formula: Due date = LMP + 280 + (cycle length − 28)

For ultrasound dating, you work backward from the scan:

Formula: Due date = scan date + (280 − gestational age at scan)

For IVF, the embryo age at transfer replaces the standard two-week ovulation offset:

Formula: Due date = transfer date + (266 − embryo age)

A day-3 transfer adds 263 days. A day-5 blastocyst adds 261. A day-6 blastocyst adds 260.

For a known conception date:

Formula: Due date = conception date + 266 days

Once the due date is set, the LMP is due date minus 280 days, and conception is due date minus 266. Gestational age today is the number of days between LMP and today, formatted as whole weeks plus the remainder in days. Cycle length is clamped between 20 and 45 days. Gestational days at scan are clamped between 0 and 6. The tool flags any gestational age above 42 weeks or below 0 as a likely input error.

Worked Example

Suppose your LMP started on January 20, 2026, and you’re checking on March 12, 2026. Your cycles run 30 days.

LMP: January 20, 2026
Days from LMP to today: 51 days
Gestational age: 7 weeks 2 days
Due date = January 20 + 280 + (30 − 28) = October 29, 2026
Estimated conception = January 20 + (30 − 14) = February 5, 2026
Trimester: First
Days remaining: 231
Weeks remaining: 33 weeks 0 days

The calculator would show “7 weeks 2 days” in the hero panel, October 29, 2026 as your due date, and February 5, 2026 as the estimated conception date. The milestone table would flag your next appointment window (the end of the first trimester at 13 weeks 6 days) as June 3, 2026. The anatomy scan around 20 weeks would land on June 30, 2026.

If an early ultrasound instead put you at 7 weeks 5 days on the same scan date, that’s a 3-day difference from the LMP estimate. It falls under the 5-day redating threshold before 9 weeks, so most clinics would keep the LMP-based due date. Anything over 5 days would trigger a switch to the ultrasound-based date.

Frequently Asked Questions

What is gestational age?

Gestational age is the number of weeks and days since the first day of your last menstrual period. It’s the standard dating system in obstetrics and runs about two weeks ahead of fetal age, since ovulation happens roughly two weeks after the LMP.

How do you calculate gestational age from LMP?

Count the days from the first day of your last period to today, then divide by seven. The remainder is the day count. A pregnancy 60 days from LMP is 8 weeks 4 days.

What is the difference between gestational age and fetal age?

Gestational age counts from the LMP. Fetal age counts from fertilization, about two weeks later. A 12-week gestational pregnancy is roughly 10 weeks fetal age.

How accurate is ultrasound dating for gestational age?

A first-trimester ultrasound before 14 weeks is accurate to within 3 to 5 days. Second-trimester dating runs 1 to 2 weeks. Third-trimester dating is less precise because fetal growth varies.

How is gestational age calculated for IVF?

IVF uses the embryo age at transfer. The due date is transfer date + (266 − embryo age). A day-5 blastocyst adds 261 days; a day-3 transfer adds 263; a day-6 blastocyst adds 260.

What are the three trimesters?

First trimester runs 0 to 13 weeks 6 days. Second trimester runs 14 to 27 weeks 6 days. Third trimester starts at 28 weeks and continues to delivery.

What does “term” mean in pregnancy?

Early term is 37 to 38 weeks 6 days. Full term is 39 to 40 weeks 6 days. Late term is 41 to 41 weeks 6 days. Post-term starts at 42 weeks.

Can gestational age change during pregnancy?

Yes. If an early ultrasound disagrees with the LMP-based estimate by more than the redating threshold, ACOG recommends switching to the scan-derived date. Once redated, gestational age updates to match.

Why is gestational age counted from LMP instead of conception?

The LMP is observable and datable. Ovulation and fertilization usually happen silently. Counting from the LMP gives every provider, lab, and screening protocol a shared reference point.

How many weeks is a full-term pregnancy?

Full term is 39 to 40 weeks 6 days of gestational age, which is about 37 to 39 weeks of fetal development. Only about 4% of babies are born on their exact due date.

How do I calculate my due date from gestational age?

Add (280 − current gestational age in days) to today. If you’re at 12 weeks 0 days (84 days), the due date is 196 days from today.

What if my gestational age doesn’t match my LMP?

That usually means ovulation happened later or earlier than the standard day 14 assumption. Your provider may redate the pregnancy based on an early ultrasound, which becomes the new official dating.

Disclaimer

This gestational age calculator is for educational purposes only and is not medical advice. It applies standard dating formulas to the inputs you provide. Actual clinical dating involves ultrasound measurements, provider judgment, and sometimes redating based on scan results. Always rely on your healthcare provider’s dating for decisions about screening windows, delivery timing, or any other clinical matter.