Gestational Age Calculator: Weeks, Days & Trimester

Gestational Age Calculator

Find your current pregnancy age in completed weeks and days, plus trimester, estimated due date, and days remaining. Dates from last menstrual period, due date, ultrasound scan, or conception day.

📌Real Pregnancy Presets

📝Pregnancy Dating Inputs

Dating method

As-of date (today)

The day you want the gestational age computed for.

LMP: first day of last period

Used when dating method is LMP.

Average cycle length (days)

Adjusts LMP if your cycle is not 28 days.

Estimated due date (EDD)

Used when dating method is due date.

Ultrasound scan date

Date the scan was performed.

GA at scan: weeks

GA at scan: extra days

Conception or transfer date

Used when dating method is conception.

Result precision

Gestational age 0w 0d completed weeks and days

Trimester – based on current week

Estimated due date – LMP-equivalent + 280 days

Days remaining 0 until estimated due date

🔢Key Milestones Snapshot

280Days full pregnancy

24wViability threshold

37wStart of term

40wDue date week

📅Trimester Ranges

Trimester	Start	End	Weeks	What Happens
First	0w 0d	13w 6d	0–13	Organs and heartbeat form
Second	14w 0d	27w 6d	14–27	Movement, anatomy scan
Third	28w 0d	40w 0d	28–40	Rapid growth to birth

📈Weekly Milestones

Week	Stage	Milestone	Trimester
4–5	Embryo	Missed period, positive test	First
6–7	Embryo	Heartbeat visible on scan	First
11–13	Fetus	Nuchal screening window	First
18–22	Fetus	Anatomy scan, sex often seen	Second
24	Fetus	Viability threshold	Second
28	Fetus	Third trimester begins	Third
37–38	Newborn	Early term reached	Third
39–40	Newborn	Full term, due date near	Third
41–42	Newborn	Late and post-term	Third

📏GA to Fetal Size Reference

Week	Length	Weight	Compared To
8	1.6 cm	1 g	Raspberry
12	5.4 cm	14 g	Lime
16	11.6 cm	100 g	Avocado
20	25.6 cm	300 g	Banana
24	30 cm	600 g	Ear of corn
28	37.6 cm	1.0 kg	Eggplant
32	42.4 cm	1.7 kg	Squash
36	47.4 cm	2.6 kg	Romaine head
40	51.2 cm	3.4 kg	Small pumpkin

🗂Term Classification Grid

Class	GA Range	Weeks	Days	Timing	Note
Previable	Under 24w 0d	0–23	0–167	Very early	Below viability
Extreme preterm	24w 0d–27w 6d	24–27	168–195	Very early birth	Intensive care
Moderate preterm	28w 0d–33w 6d	28–33	196–237	Early birth	Often needs support
Late preterm	34w 0d–36w 6d	34–36	238–258	Just before term	Usually milder
Early term	37w 0d–38w 6d	37–38	259–272	Term begins	Considered term
Full term	39w 0d–40w 6d	39–40	273–286	Ideal window	Due date here
Late term	41w 0d–41w 6d	41	287–293	Past due date	Monitoring rises
Post-term	42w 0d and over	42+	294+	Overdue	Induction discussed

⚙Full Formula Breakdown

LMP methodGA days = as-of date – LMP. Cycle adjusts it: effective LMP = LMP + (cycle – 28) days, since ovulation shifts with cycle length.

Weeks and daysweeks = floor(GA days / 7); days = GA days mod 7. So 74 days becomes 10 weeks 4 days.

From due dateLMP-equivalent = EDD – 280 days. Then GA = as-of date – LMP-equivalent.

UltrasoundLMP-equivalent = scan date – (scan weeks × 7 + scan days). Then GA today = as-of date – LMP-equivalent.

ConceptionGA = (as-of date – conception date) + 14 days, because conception is about 2 weeks after LMP.

TrimesterWeeks 0–13 first, 14–27 second, 28 and up third. Uses the completed-week count.

Term statusUnder 37 weeks preterm, 37–42 term, over 42 post-term. Compared against the current GA.

Due date and remainingEDD = LMP-equivalent + 280 days. Days remaining = EDD – as-of date (negative once past due).

📋Method Reference

Method	You Enter	LMP-Equivalent	Best When
LMP	First day of last period	LMP ± cycle shift	Regular cycles, known date
Due date	Estimated due date	EDD – 280 days	Provider gave an EDD
Ultrasound	Scan date and GA at scan	Scan date – scan GA	Most accurate dating
Conception	Conception or transfer day	Conception – 14 days	Known ovulation or IVF

💡Gestational Age Tips

Dating tip: A first-trimester ultrasound is the most reliable way to date a pregnancy. If your scan GA differs from your LMP by more than about a week, providers usually re-date from the scan.

Cycle tip: The LMP formula assumes a 28-day cycle and ovulation on day 14. Longer cycles push ovulation later, so this tool subtracts or adds the difference to keep the gestational age realistic.

This gestational age calculator gives an informational estimate only and is not medical advice. Confirm dating, trimester, and due date with your own healthcare provider.

Then there’s that day (or morning) when it all becomes real. It happens when you pick up a stick and see two pink lines. Suddenlly, what happened yesterday doesn’t matter; the calendar matters now. A date you wrote off as small detail is now a center point for other dates to follow.

It’s like flipping a switch and setting a countdown timer, except without a manual from anyone. How do we calculate our gestational age? Do we calculate it by taking away some dates in the app? No, no; that won’t cut it. We’ve got to get into it: what does a week mean, exactly, such that someone can say “week 14” and the number will actualy make sense?

How to Calculate Your Due Date

This traditional approach involve your last menstrual period‘s first day. Yes, you weren’t pregnant back then, it seems backwards! But because ovulation isn’t exactly easy to pinpoint, that’s the fixed starting point used by medical community. From there, it take about 280 days, or approximately forty weeks, to reach full-term (on average).

Plug in your starting date here and the calculator will do the rest. You won’t have to worry about getting confused with timezones or adding one too many zeros when doing the math yourself. It’ll even adjust for how long your cycle typically run. This is an important adjustment that most basic calculators don’t consider.

Your cycles are longer than the default twenty-eight day standard? Then you’re ovulating later in the cycle. This also means that conception was later in line than the default model takes into account. By adding those additional days to your equation, you ensure your due date doesn’t get pulled forward too far. This lets you make your own expectations and plan to match. Just tell it what your average cycle length is and it adjust the baseline accordingly. That way the calculated due date lines up with your actual biology instead of some generic average.

But there’s also another window into time. The one provided by ultrasound scans. Ultrasounds provide a snapshot in time, measuring size of fetal structures relative to established growth curves. Because all embryos grow relatively similarly within the first 12 weeks, early ultrasounds tend to be very accurate. Afterward, other factors like genetics and environment begin to affect size, making variations more frequent.

Depending on how far off your period-based date is from an early ultrasound, doctors will sometimes change their official record dates to match the ultrasound. You can enter that ultrasound information here to backtrack your pregnancy timeline, so it matches up with your medical chart.

What’s going on in there? How does it all fit? Understanding the trimester breakdown can help put things into perspective. Week one through 13 sees quick cell growth and organs takes shape. After week 14 we enter the second trimester (and for a lot of parents this is when things start feeling stable). This is where movement typically occur and parents gets their anatomy scan, which shows how the body is built. These parameters will give you an idea of when each test will happen. So if you don’t want to memorize the dates, know the big picture so your prenatal appointments aren’t as confusing or disconnected from each other, just part of a larger journey.

After that it’s all about milestones in the home stretch. Once you hit 37 weeks, you’re technically “in term” with a baby who could be born now, but might still have some time before reaching her due date. And full term, the sweet spot many people think about as the best time to go into labor, lands between 39-40 weeks. After 42 weeks, your pregnancy enters post-term land, when you’ll need more frequent check-ins. Those designations is important because they factor into decisions like interventions and when to induce. All that information is neatly spelled out on the page, so you know whether you’re ahead or behind each clinical milestone.

Sometimes we fall into the trap of fixating on a particular day, but as people we’re not so predictable. In fact, only around 5% of babies hits their assigned due date with absolute accuracy. Most will land somewhere in the ballpark of a couple of weeks on either side of that predicted date. Yes, it’ll have a nice round figure attached which you can tick off along the way but really what it means is you could of been both informed and ready. Not only do you have an idea where you stand but also why the dates change. That’s a basis for your health care team. That understanding matters far more then any exact date ever can.