How Do You Know How Far You Are In Pregnancy? | Dates That Fit

That “How many weeks am I?” moment hits fast. You want a straight answer you can use for appointments, screening windows, and planning. The good news: there are only a few reliable ways clinics date a pregnancy, and you can do a solid first estimate at home in minutes.

This article shows the week-counting basics, when an ultrasound changes the date, and how to stop second-guessing once you have an official due date.

What “Weeks Pregnant” Means In Practice

Most clinics use gestational age, counted from day one of the last menstrual period (LMP). Conception usually happens later, yet LMP dating is the standard because many people can recall a period start date, and it lines up with care schedules.

When people say “40 weeks,” they mean 40 weeks from LMP, not 40 weeks from conception. Many births happen between 37 and 42 weeks. The goal is one shared calendar, not a perfect guess about the exact day fertilization happened.

How Do You Know How Far You Are In Pregnancy? Week Counting Basics

If you know your LMP date, you can estimate your weeks pregnant like this:

1. Count days since the first day of your last period.
2. Divide by 7 to get weeks, then keep the leftover days.

Example: 48 days since LMP equals 6 weeks + 6 days (often written as 6w6d). Many appointment windows are set in that same “weeks plus days” format.

If your cycles vary a lot, or you don’t know your LMP, you can still get a usable estimate using test timing, ovulation tracking, or an ultrasound. Start with the clues you already have.

Start With The Clues You Already Have

• Last period start date: write it down, even if it’s a best guess.
• Typical cycle length: your usual range over the last few months.
• First positive test date: helps set a lower bound for LMP-based weeks.
• Ovulation tracking: LH surge date, basal body temperature shift, or tracked ovulation symptoms.
• Fertility treatment dates: insemination, egg retrieval, or embryo transfer date, if relevant.

Bring these notes to your first visit. It reduces confusion when your app’s due date and your clinic’s due date don’t match.

Using LMP With A Typical 28-Day Cycle

If your cycles are close to 28 days, LMP dating often works well as a first estimate. Many tools add 280 days (40 weeks) to the LMP date to get an estimated due date.

The NHS has a due date calculator based on LMP, which can be handy once you have your dates written down.

Adjusting For Longer Or Shorter Cycles

If your cycles are steady but not 28 days, you can shift the basic estimate. Add the difference between your usual cycle length and 28 days.

Cycle usually 32 days? Add 4 days to the LMP-based due date. Cycle usually 26 days? subtract 2 days. This is a rough correction, not a final diagnosis. An early scan can become the anchor if your cycles are irregular.

When Your Only Anchor Is A Positive Test

A first positive home test can still help you sketch a week range. Many people turn positive around the time a period is missed. If you know your first clear positive date, it often suggests you were at least around 3 to 4 weeks pregnant by LMP dating at that time.

Use that as a guardrail, then share the test date with your clinician. It helps decide whether an early scan is likely to show what you expect.

Dating By Ultrasound And Why Timing Changes Accuracy

Ultrasound dating estimates gestational age using measurements. Early in pregnancy, growth tends to be more consistent across pregnancies, which makes early measurements tighter for dating.

ACOG explains how clinicians set gestational age and the estimated due date using LMP, ultrasound, or both, and why the first accurate ultrasound is often the best anchor. Read: ACOG “Methods for Estimating the Due Date”.

On a first-trimester scan, crown–rump length (CRL) is commonly used for dating. Later in pregnancy, other measurements are used and the dating window is wider.

The World Health Organization also notes the value of at least one ultrasound before 24 weeks for gestational age assessment and related checks. See: WHO guidance on ultrasound before 24 weeks.

Why Two Different Due Dates Can Both Seem Right

One due date can come from LMP. Another can come from an ultrasound. That mismatch is common, especially with irregular cycles or later ovulation.

LMP dating assumes a typical ovulation pattern. Ultrasound dating estimates age from size at the time of the scan. If ovulation happened later than average, the scan may shift the due date later. If ovulation happened earlier, it may shift it earlier.

After your clinician documents an estimated due date based on LMP and/or the first accurate ultrasound, treat that date as the anchor for week counts, test windows, and planning. Update your app to match.

Pregnancy Dating Methods And When Each Works Best

This table summarizes the most common dating methods and what each one does well.

Dating method	Best use case	What to watch for
LMP (last menstrual period)	Regular cycles, clear period start date	Cycle swings can shift ovulation timing
Cycle-length adjusted LMP	Steady cycles that are not 28 days	Still a rough estimate until an early scan
Known conception date	Single sex date with no other exposures	Fertilization may occur days later
Ovulation tracking (LH or BBT)	People who tracked ovulation	LH surge is a cue, not the exact moment
Early ultrasound (CRL)	Unclear LMP, irregular cycles, date mismatch	Earlier scans tend to date more tightly
Second-trimester ultrasound	No early scan available	Wider dating window than first trimester
IVF or embryo transfer dating	Assisted reproduction with known dates	Calculated from transfer day and embryo age
hCG blood test trends	Early uncertainty with lab follow-up	Complements dating, doesn’t replace it

A Clean Home Workflow Before Your First Appointment

If you want a calculator that matches a standard LMP approach, the NHS pregnancy due date calculator is a clear option for a one-time check.

1. Write your LMP date. If unsure, note it as a best guess.
2. Note your usual cycle length. If it’s steady, adjust from the 28-day baseline.
3. Mark your first positive test date. Use it to sanity-check your estimate.
4. Add ovulation data if you tracked it. An LH surge date is useful context.
5. Pick one due date and stop re-checking daily. Bring your notes to the clinic, then switch to the documented due date.

If You Had IVF Or Another Fertility Treatment

For IVF pregnancies, gestational age is calculated from embryo age and transfer date. Bring your transfer date and embryo age (day 3, day 5, and so on). That’s often the cleanest dating anchor because it’s documented.

If Your Periods Are Irregular Or You Recently Stopped Hormonal Contraception

After stopping hormonal contraception, cycles can be unpredictable for a while. In that scenario, LMP dating may be shaky. An early ultrasound may become the primary anchor, with LMP kept as background context.

Signs Your Week Count Might Be Off

• Cycles that vary widely month to month
• Bleeding that could have been mistaken for a period
• A delayed positive test after a missed period
• Ovulation tracking that points to a later ovulation date
• A scan estimate that lands more than several days away from your LMP estimate

No symptom can date a pregnancy down to the day. If your dates feel messy, that’s normal. Bring your notes, and let the scan and clinical record settle the timeline.

When You Should Get In Touch With A Clinician Soon

Some symptoms call for prompt medical care, especially early in pregnancy:

• Heavy bleeding or bleeding with clots
• One-sided pelvic pain, shoulder pain, or fainting
• Severe abdominal pain
• Fever with pelvic pain
• Symptoms that feel sudden or alarming

Clinicians may use ultrasound and lab tests to check how the pregnancy is developing and to confirm location and dating.

Situation	What usually helps	What to bring
Unsure LMP date	Early ultrasound dating	Best-guess LMP, cycle history, test dates
Irregular cycles	Ultrasound anchor + clinical notes	Recent cycle lengths, any tracking data
Scan and LMP differ	Use the first accurate scan for the official date	Prior scan reports, LMP date, tracking notes
IVF pregnancy	Transfer-based dating	Transfer date, embryo age, clinic summary
Bleeding or pain early on	Clinical exam, ultrasound, lab tests	Bleeding notes, pain timing, meds list
Late first prenatal visit	Ultrasound plus care plan catch-up	Any prior tests, questions list

Keep One Official Date And Let The Calendar Do Its Job

Once your clinician documents an estimated due date, use it in all your places. Put it in your calendar, update your app, and count weeks from that single anchor.

If a later scan shows a different week estimate, don’t self-redate. Later growth differences are common and don’t always reflect a different start date. Your medical record will keep using the established due date unless your clinician documents a rare change.