How Far Along Am I In Pregnancy Calculator | Find Your Week

If you’ve ever stared at a positive test and thought, “Wait… what week does that make me?” you’re not alone. Dating a pregnancy can feel odd because the clock starts before you were pregnant. A “how far along” calculator turns a few dates into a clear answer: your pregnancy week, your gestational age in days, and an estimated due date.

This article shows what these calculators use, how to enter your details without guesswork, and how to sanity-check the result. You’ll also see when an ultrasound date should take the lead, plus a clean checklist you can save.

How Far Along Am I In Pregnancy Calculator: What It Uses

Most online tools use one of three starting points. The goal stays the same: estimate gestational age, which is the week-and-day format used in clinics and on most pregnancy charts.

Last menstrual period date

This is the most common input. You enter the first day of your last menstrual bleed. The calculator counts forward in weeks and days and places your due date 280 days after that date (40 weeks). ACOG describes the 280-day convention in its clinical guidance on due-date dating.

This method assumes ovulation around day 14 of a 28-day cycle. If your cycles run longer or shorter, many calculators let you adjust cycle length so the estimate fits better.

Conception or ovulation date

If you tracked ovulation with test strips, basal temperature, or you have a known conception date, a calculator can count from that point and then convert to gestational age. It usually adds 14 days, since the standard dating system starts two weeks earlier than ovulation for a typical cycle.

This can feel counterintuitive at first. It’s also why two people who conceived on the same day can report different “weeks pregnant” if one app uses LMP rules and the other uses ovulation-based rules.

IVF transfer date

With IVF, dating can be clean because embryo age is known. Many IVF calculators ask for transfer date and whether the embryo was day-3 or day-5 at transfer, then back-calculate a gestational age that matches clinical dating.

If you’re using an IVF calculator, stay consistent about embryo age at transfer. A day-5 blast transfer and a day-3 transfer will not map to the same gestational day, even if the calendar dates look close.

Gestational age vs fetal age

You’ll see two “ages” used in pregnancy writing. Gestational age is what clinics use. Fetal age is closer to time since conception. Fetal age is usually about two weeks less than gestational age, but cycle timing can shift that gap.

If a calculator says you’re 10 weeks pregnant, that often means about 8 weeks since conception for someone with ovulation near day 14. Once you know which clock you’re reading, the rest of the dating language makes more sense.

How to get the most accurate result from a calculator

A calculator can only work with what you enter. A few small choices can make the output line up better with your real timing.

Use the first day of bleeding for your last period

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Set your usual cycle length

If your cycle is 32 days most months, enter 32 instead of 28. If it varies, use your average from the last 3–6 cycles. A range is normal, so aim for “typical,” not “perfect.”

Cycle length matters because it changes the default ovulation assumption. Longer cycles often mean later ovulation. Shorter cycles often mean earlier ovulation. The week count can shift by days based on that one setting.

Know what a “missed period” means for your cycle

Many people test after a missed period and assume they’re “four weeks pregnant.” That can be close for a 28-day cycle. With longer cycles, the “missed period” point arrives later, so the calculator’s week count may look smaller than you expected.

Try not to judge the number against how you feel. Symptoms vary a lot from person to person and can’t date a pregnancy reliably.

When an ultrasound estimate should take the lead

Early ultrasound measurement is often used to confirm dating, especially when cycles are irregular or LMP is unclear. ACOG outlines how clinicians use LMP and ultrasonography together, plus when a scan-based estimate can replace an LMP-based estimate in Methods for Estimating the Due Date.

If you have a scan report that lists “GA” (gestational age) as weeks + days, that can be used as the anchor for your current week count. From there, calculators simply count forward to today.

How Far Along In Pregnancy Calculator With Irregular Cycles

If your cycle swings from month to month, LMP math can land wide of the mark. That doesn’t mean the calculator is “wrong.” It means LMP is a shaky anchor when ovulation timing shifts.

In that situation, two inputs tend to work better:

• Ovulation date from tracking (if you have it)
• Ultrasound gestational age from an early dating scan

If you don’t have either, use LMP for a starting estimate, then plan to switch to the clinic’s dating once it’s set. That keeps appointment timing and lab timing aligned with how care is scheduled.

Common reasons your week count looks “off”

Seeing a number that doesn’t match what you expected can be unsettling. Most of the time, it’s just math colliding with real-life cycles.

Recent hormonal contraception

After stopping hormonal contraception, cycles can take time to settle. Ovulation can happen earlier or later than your old pattern, so an LMP-based estimate can drift.

Bleeding that wasn’t a true period

Some early pregnancy bleeding gets mistaken for a period. If you bled lightly for a day or two, it may not be a full cycle reset. A scan-based date may fit better once you have it.

Later ovulation than the calculator assumes

If you ovulated on day 20 rather than day 14, an LMP tool will label you further along than you are by about six days. That’s enough to change which week-by-week page you land on.

Test date confusion

Some tools offer “I took a test on…” inputs. A test date can’t date a pregnancy by itself. It only sets a latest possible window based on hormone rise. If a calculator asks for a test date, treat it as a helper, not the anchor.

Inputs you may see in pregnancy calculators

Different sites ask for different details. Use this table to pick the right starting point and avoid entering a number that feels right but is based on the wrong definition.

Calculator input	When to use it	What it changes
LMP first day	You know the first day of your last menstrual bleed	Sets gestational age and due date from the 40-week standard
Average cycle length	Your cycles are usually longer or shorter than 28 days	Shifts the ovulation assumption and nudges the estimate
Conception date	You tracked ovulation or have a known conception window	Adds 14 days to align with gestational dating
Ovulation date	You used ovulation tests or basal temperature tracking	Builds a week count from ovulation and converts to gestational age
IVF transfer date	You had an embryo transfer	Back-calculates start date based on embryo age at transfer
Ultrasound gestational age	You have a scan report with weeks + days	Lets the tool project forward to “today” and estimate due date
Ultrasound date	You know the date of the scan	Pairs with scan age to calculate your current week
Due date from clinic	You were given an estimated due date already	Counts backward to show current gestational age

How to calculate your week in two minutes

Even if you plan to use a calculator, knowing the rough math helps you trust the output.

1. Start with LMP day one. Count forward to today. Each full 7-day block is a week.
2. Add the leftover days. Weeks + days is your gestational age.
3. Project to 40 weeks. Add 280 days from LMP day one for an estimated due date.

Sample: if your LMP day one was a Monday and today is 23 days later, you’re 3 weeks 2 days by LMP dating.

If you’re using a conception date, count forward from that day, then add 14 days to match gestational dating. For IVF, use the transfer date and embryo age at transfer, then convert to gestational age.

What your current week helps you plan

People use week counts for more than curiosity. It helps line up appointments, screening windows, and the “what happens next” moments that come up in prenatal care.

Trimesters at a glance

Trimester cutoffs vary by source, but the three-part structure is widely used in public health writing. The CDC’s pregnancy information hub gives a broad overview of pregnancy timing and how care is commonly organized across pregnancy: CDC pregnancy information.

Due date is a planning target

A due date is a target used for planning and monitoring. Birth often happens in a window around that date, and your care team may talk about what “term” means in weeks. If you’re planning work leave, travel, or childcare help, treat the due date as the center point and build slack on both sides.

Why two calculators can disagree by a few days

One tool might say 9 weeks 2 days while another says 9 weeks 6 days. That gap usually comes from cycle-length assumptions, rounding rules, or which day the calculator counts as day one. Once your clinic sets an estimated due date, use that dating for your planning so appointment timing stays consistent.

Week-by-week checkpoints you can bookmark

Use this table as a quick map. It doesn’t replace medical advice. It gives context for what many people schedule or notice around each stretch of pregnancy.

Gestational weeks	Common timing	Notes you can use
4–5	Positive test window for many	Home tests vary; blood tests can detect earlier in some cases
6–9	First visit often scheduled	If your clinic offers early dating scans, LMP may be confirmed here
10–13	End of first trimester range	Some screening tests are offered during this window
14–20	Second trimester stretch begins	Anatomy scan timing varies by clinic and local practice
21–27	Later second trimester	Many glucose screening protocols fall near the end of this range
28–36	Third trimester stretch	Visit frequency may increase; fetal movement tracking may be discussed
37–40+	Term window used by many clinics	Labor can start any time; your care team may plan extra monitoring

How to use an ultrasound report for dating

If you have a scan report, it often includes a gestational age (GA) in weeks + days and an estimated due date (EDD). A calculator that accepts “ultrasound age on scan date” can compute your current week by counting forward from the scan date.

A clean way to enter it:

• Enter the scan date as shown on your report.
• Enter the GA listed on that same report (weeks + days).
• Let the calculator project forward to today.

If your report lists multiple measurements, use the final GA or EDD the report concludes with, not a single measurement line. If you’re unsure which line is the final one, your clinic can confirm which due date they are using for your chart.

When to switch from a web estimate to the clinic’s date

An online estimate can be a helpful starting point. Your clinic’s recorded due date becomes the reference point for labs, scan timing, and charting.

The NHS due date calculator explains how due dates are worked out from the first day of your last period and how scan dating fits into real care: how due dates are worked out.

Signs an online estimate should be treated as a rough guess

• You don’t know your LMP date within a few days.
• Your cycles vary by more than about a week month to month.
• You had bleeding that didn’t match your usual period.
• You stopped hormonal contraception and cycles are still shifting.
• You used IVF or ovulation induction medications.

How Far Along Am I In Pregnancy Calculator results you can record

Once you get a week and due date, write down a few items. This keeps your timeline consistent across apps, appointments, and notes.

• Your LMP first day (or transfer date, if IVF).
• Your current gestational age (weeks + days).
• Your estimated due date.
• The date and result of your first dating scan, if you have it.
• The due date your clinic uses after that scan.

If your apps disagree, pick one source of truth. Most people choose the clinic’s EDD once it’s set. Then the week-by-week content you read lines up with your scheduled care.

When to get urgent care right away

Online calculators are for planning and curiosity. They can’t check symptoms. If you have heavy bleeding, severe one-sided pain, fainting, shoulder pain, chest pain, trouble breathing, or you feel something is seriously wrong, seek urgent care right away.

The CDC lists urgent warning signs during pregnancy (and after birth) on its HEAR HER page: Urgent maternal warning signs and symptoms.

A clean checklist to match your calculator with real care

Use this as a final pass after you get your result.

1. Check you entered the first day of your last period, not the last day.
2. Set your typical cycle length if it’s not 28 days.
3. If you tracked ovulation, compare the LMP estimate to ovulation-based timing.
4. Once you get an early dating scan, use the due date your clinic records.
5. Store your final due date in one place so apps don’t drift apart.