How Can You Tell How Far Pregnant You Are? | Due Date Math

Figuring out how far pregnant you are gets easier once you use the same clock your clinic uses: weeks of gestational age. That single number drives scan timing, blood tests, and the way pregnancy milestones are described.

How pregnancy weeks are counted

Most pregnancies are dated from the first day of your last menstrual period (LMP). It feels backwards because conception happens later, yet LMP is a real calendar date many people can recall, while ovulation timing shifts.

Gestational age is written like “10 weeks, 3 days.” It is not the same as the baby’s age from conception. For a 28-day cycle, conception age is often about two weeks less than gestational age, since ovulation tends to occur around the middle of the cycle.

Two timelines that get mixed up

• Gestational age: counted from LMP and used in medical records.
• Conception age: counted from fertilization and used in some fertility tracking.

If you know your ovulation or conception date, a simple translation is to add 14 days, then count forward to today. People with long or short cycles can land a few days off, so treat it as a strong estimate.

How far pregnant you are by weeks and dates

You want one result: your gestational age today, plus an estimated due date. Start with the method that fits your situation, then use the cross-checks so the numbers feel sane.

Method 1: Last menstrual period dating

If you know the first day of your last period, you can estimate a due date by adding 280 days (40 weeks). The NHS due date calculator uses LMP to return an estimated due date and a week count.

Cycle length matters. The default “40 weeks” math assumes a 28-day cycle. If you often ovulate later, LMP dating can make you look further along than you are.

A quick home calculation

1. Write down the first day of your last period.
2. Count the number of days from that date to today.
3. Divide by 7 to get weeks, then use the remainder as extra days.

Method 2: Ovulation or conception dating

If you tracked ovulation with test strips or basal temperature, you may trust that window more than LMP. Convert it to gestational age by adding 14 days, then count forward.

Home ovulation tools detect patterns around ovulation, not the exact moment of fertilization. Implantation timing varies, so this method is best used as a cross-check.

Method 3: IVF or embryo transfer dating

With IVF, dating is often clear because the transfer day and embryo age are known. Your clinic usually provides a due date and a week count that stays consistent for the rest of pregnancy.

Method 4: First-trimester ultrasound dating

When dates are uncertain, an early ultrasound can set the timeline. In the first trimester, the scan measures crown-rump length (CRL), then converts it to gestational age. ACOG’s Methods for Estimating the Due Date explains how clinicians choose and document an estimated due date.

If an early scan and LMP disagree by more than a small margin, clinicians may change the due date to match the scan. Delayed ovulation is a common reason. A bleed that looked like a period can be another.

The JOGC guideline on determining gestational age by ultrasound notes that when there are multiple early scans, the earliest suitable first-trimester CRL scan is used for dating.

Method 5: Later ultrasound or belly measurements

Second-trimester scans and clinical measurements like fundal height help track growth. They can date a pregnancy when no early scan exists, yet they are less precise for setting the original due date because growth rates vary more as weeks pass.

Which method should you trust most

Think of pregnancy dating like a ladder: start with LMP, then move to better data when it exists. Many clinics set the due date once using the best information available, then keep it stable so every test window lines up.

If your cycles are regular and you’re confident about LMP, LMP dating often matches an early scan closely. If your cycles vary, you recently stopped hormonal birth control, you were breastfeeding, or you had spotting that could be mistaken for a period, the early scan tends to be the steadier anchor.

How clinics set a single due date

It helps to know what the clinic is trying to do. They’re not chasing a perfect “conception day.” They’re choosing one due date that keeps every time-sensitive step on the right week: screening bloodwork, anatomy scan timing, and decisions near the end of pregnancy.

When LMP is reliable and an early scan lines up, the due date often stays exactly where it started. When dates are fuzzy, an early scan can become the starting point. After that, later growth scans usually do not reset the due date. They’re used to check growth trends against the due date that was already set.

If you get a new due date after a scan, ask for two numbers in writing: your estimated due date and your gestational age on the scan day. With those, you can calculate your week and day count on any date without guessing.

Common ways to date a pregnancy and where each fits

Dating method	What you need	Best fit
Last menstrual period (LMP)	First day of last period	Regular cycles; first estimate
Cycle length adjustment	Average cycle length	Consistent cycles that aren’t 28 days
Ovulation tracking	LH tests or temperature shift	Useful cross-check for timing
Known conception timing	Single-event timing (rare)	When timing is unusually certain
IVF or embryo transfer	Transfer date and embryo age	Calendar-based dating for IVF
First-trimester ultrasound (CRL)	Early scan measurement	Strong anchor when LMP is uncertain
Second-trimester ultrasound	Later scan measurements	Dating when no early scan exists
Fundal height	Clinic measurement	Growth tracking after dating is set

How to sanity-check your week count

Before your first scan, two quick checks can prevent a lot of confusion.

Check your first positive test date

A positive home test near your missed period often lines up with a gestational age around four weeks. If your LMP math puts you at eight weeks on the day you first tested positive, recheck your LMP date and consider whether an earlier bleed was not a true period.

Check your due date with a calendar rule

Naegele’s rule is a classic shortcut: take the first day of LMP, subtract three months, add one year, then add seven days. Johns Hopkins’ steps for calculating a due date lay it out clearly.

If your calculator gives a different result, confirm you used the first day of bleeding, not the last day. It’s a common slip.

Why your dates can change after a scan

A dating scan measures size and maps that size to an age curve. Early on, embryos grow at similar rates, so size and age match well. Later, growth spreads out, so size can’t lock in age as tightly.

If a scan gives you a new due date, write it down and use it for everything that follows. Mixing two due dates is a fast route to missed windows and stress.

Situations that can make pregnancy dating tricky

Situation	Why it shifts dates	What tends to help
Irregular cycles	Ovulation timing varies	Early ultrasound dating (CRL)
Stopped hormonal birth control	First cycles may be longer or shorter	LMP plus early scan to confirm
Breastfeeding	Ovulation can return unpredictably	Scan-based dating
Light bleeding or spotting	Bleeding can be mistaken for a period	Scan dating and symptom timeline
After miscarriage or recent pregnancy	Cycles may not have reset fully	Scan dating plus clear history
IVF or transfer	Dates are based on embryo age and transfer day	Use clinic-provided due date
LMP and scan disagree	Delayed ovulation is common	Use earliest suitable CRL scan
Late first scan	Later scans date with less precision	Use best available scan, then keep it stable

When symptoms matter more than date math

If you have heavy bleeding, severe one-sided pain, fainting, fever, or pain that feels wrong, contact an urgent maternity service right away. Early pregnancy can include conditions that need fast assessment, like ectopic pregnancy.

A quick recap you can use today

Start with LMP and count your weeks and days. If your dates are uncertain, an early ultrasound measurement often sets the clearest due date. Once a due date is set, keep it consistent in your notes so every week count lines up.