How Do You Tell How Far Pregnant You Are? | Weeks Made Clear

If you’ve ever stared at a calendar and thought, “Wait… what week am I even in?” you’re not alone. How do you tell how far pregnant you are when the answer seems to depend on dates you may not remember, cycle lengths that vary, and scan results that can shift the timeline?

The good news: there’s a standard way pregnancy is dated, and once you know the rules, it becomes simple to track. This article walks you through the exact steps people use in real clinics, plus the common traps that throw the count off.

What “How Far Pregnant” Means In Real Terms

When most people ask how far pregnant they are, they mean “How many weeks along am I?” That week count is called gestational age. It’s used for due dates, scan timing, blood tests, and week-by-week milestones.

Here’s the part that surprises many first-time parents: gestational age is usually counted from the first day of your last menstrual period (often shortened to LMP). That is the standard reference point used in obstetrics, even though conception often happens later.

So your “weeks pregnant” number can be ahead of the actual time since fertilization. That’s normal. It’s also why someone can be “4 weeks pregnant” before they’ve even had a positive test for very long.

How Do You Tell How Far Pregnant You Are? Using Dates And Scans

Most people land on their pregnancy week using one of these starting points:

• Last menstrual period (LMP): Count forward from day 1 of your last period.
• Early ultrasound: A first-trimester scan can date the pregnancy using fetal measurements.
• Conception or ovulation date: Useful if you tracked ovulation closely, but it still gets translated into “gestational” weeks.
• IVF dates: Transfer day is known, so dating can be pinned down tightly.

Clinics commonly start with LMP, then confirm or adjust based on the first reliable ultrasound. ACOG describes setting an estimated due date once LMP data and the first accurate ultrasound are available, then keeping that due date consistent except in unusual cases. ACOG’s Methods for Estimating the Due Date lays out that approach.

Step 1: Find The First Day Of Your Last Period

If you remember only one date, make it this one: the first day you had full flow bleeding for your last normal period. Spotting can confuse things, so think “real period start,” not a day of light smears.

Write it down. Put it in your phone. If you use a period tracker, check the entry and confirm it matches what you recall.

Step 2: Count Pregnancy Weeks From That Date

Gestational age is counted in weeks and days, like “9 weeks 3 days.” Day one is the first day of your last period. Week one runs through day seven, and so on.

If you want the classic due-date math, the standard estimate is 40 weeks (280 days) from LMP. That rule is built into many clinical tools and calculators.

Step 3: Adjust For Cycle Length If Yours Is Not 28 Days

The 40-week estimate assumes a 28-day cycle with ovulation near day 14. If your cycles run longer or shorter, ovulation may shift. That can make an LMP-based estimate feel “off” compared with when you think conception happened.

Many clinicians still start with LMP because it’s a consistent anchor, then they compare it to early ultrasound dating. That scan often settles the question when cycles are irregular.

Step 4: Use An Early Ultrasound To Confirm Dating

An early ultrasound usually dates pregnancy using measurements like crown-rump length in the first trimester. That’s the scan most likely to refine your week count when LMP is uncertain or cycles vary.

For a technical overview of ultrasound dating methods, measurement timing, and how clinicians assign due dates, see the American Institute of Ultrasound in Medicine handout on pregnancy dating. AIUM handout on ultrasonography for dating pregnancies summarizes how dating is derived from scan measurements.

Step 5: Keep One “Official” Due Date Once It’s Set

It’s tempting to recalculate every time you see a new app estimate or a friend says, “You look farther along.” The clinic’s chart will rely on a single estimated due date tied to the best dating data early in pregnancy, since later measurements are less useful for dating.

Telling How Far Along You Are With LMP And Ultrasound

If you want a simple, real-world rule: use LMP for a first estimate, then treat the first good ultrasound as the tie-breaker when dates don’t line up.

If you like calculators, a reputable public-health service can be a clean starting point. The UK’s NHS due date calculator uses the first day of your last period and notes that pregnancy length can vary across a wide week range. NHS pregnancy due date calculator is a straightforward example of the LMP-based method.

Still, a calculator is only as good as the date you enter. If you are unsure of your last period date, that uncertainty should be shared with your maternity team so scan dating can carry more weight.

Common Reasons Your Week Count Feels “Wrong”

Plenty of people feel a mismatch between what they “think” their week should be and what the clinic records. Here are the usual culprits:

• Irregular cycles: If ovulation shifts, LMP estimates can be off.
• Not sure what counts as LMP: Spotting or breakthrough bleeding can be mistaken for a true period.
• Recent hormonal contraception changes: The first cycle off birth control can be odd.
• Breastfeeding or recent pregnancy: Cycles can be unpredictable.
• Late first scan: Dating is easiest earlier, and less clean later.

Also, language causes confusion. People sometimes mix up gestational age (weeks since LMP) and “fetal age” (time since conception). Many medical references stick with gestational age as the shared standard. The Merck Manual explains gestational age as the number of weeks from the first day of the last normal menstrual period. Merck Manual’s definition of gestational age matches how weeks are recorded in clinical care.

Dating Methods Compared Side By Side

Use this table to see which input fits your situation and what you can realistically expect from it. The goal is a stable week count you can use for appointments and test windows.

Dating Method	What You Need	Where It Works Best
LMP counting	First day of last normal period	Regular cycles and clear period dates
Cycle-length adjusted LMP	LMP plus typical cycle length	Consistent cycles that are not 28 days
First-trimester ultrasound	Early scan measurements	Irregular cycles or uncertain LMP
Second-trimester ultrasound	Scan measurements later in pregnancy	When early scan was missed
Known ovulation date	LH testing or temp tracking date	Careful tracking with a clear surge pattern
Known conception date	Single unprotected intercourse date	Only when timing is truly narrow
IVF transfer dating	Transfer date and embryo age	IVF cycles with documented timing
Clinic-set EDD	LMP plus first reliable ultrasound	Most routine prenatal care plans

How To Calculate Your Current Week In Daily Life

Once you have a starting point (LMP or scan-based gestational age), you can calculate your current week without guesswork.

If You Have An LMP Date

1. Start at the first day of your last normal period.
2. Count full weeks to today’s date.
3. Count extra days after the last full week.

Example format: “12 weeks 5 days.” That’s how most clinics chart it.

If Your Clinic Gave You A Gestational Age On Scan Day

1. Write down the scan date.
2. Write down the gestational age given on that date.
3. Add the number of days since the scan.

This method is handy if you never had a clear LMP or if your due date was set from early ultrasound. It also prevents you from chasing new estimates each week.

When LMP And Ultrasound Don’t Match

Mismatch happens. The practical question is: which date should you trust for planning? In many care settings, early ultrasound carries more weight when the gap is large or when the LMP date is uncertain.

ACOG describes establishing gestational age and due date using LMP, the first accurate ultrasound, or both, then documenting it clearly. That pattern is meant to keep care consistent across scans, labs, and appointments. ACOG’s due date guidance is often cited in clinical policy on dating.

If you’re looking at your own numbers, this is the calm way to think about it: a small difference can come from ovulation timing or recall error. A larger difference often points to an LMP that wasn’t the true starting point for the cycle that led to pregnancy.

What To Track After You Know Your Week

Knowing your week isn’t just trivia. It changes what your care team schedules and when certain results are interpreted. Once you have a stable due date and a week count, these are the items worth tracking in a notes app:

• Your clinic’s estimated due date (EDD)
• Your current gestational age format (weeks + days)
• Date and results of your first dating ultrasound
• Your typical cycle length (if known)
• Any bleeding episodes that might be confused with LMP

This is also a good time to pick one “source of truth” for your week count. Your clinic record is usually the one that drives medical timing.

Timing Checks You Can Use Week By Week

Here’s a practical reference for what your week count usually controls. This is not a full prenatal schedule, since timing varies by country, clinic, and risk factors. It’s a quick way to see why the dating method matters.

Timing Anchor	What It Influences	Why Week Accuracy Helps
Dating scan window	Setting the clinic’s official due date	Early measurements line up best with gestational age
First trimester screening windows	Blood tests and scan-based screening timing	Some results depend on gestational week
Anatomy scan timing	Detailed fetal structure review	Scheduling hinges on a reliable week count
Growth checks later in pregnancy	Tracking fetal size trends	Trend interpretation needs consistent dating
Viability and preterm definitions	How “weeks” are used in medical notes	Gestational age is the shared clinical language
Due date planning	Work plans, childcare, travel limits	A stable EDD keeps plans grounded

Fast Self-Check: Which Method Fits You Today?

If you want a quick decision path, use this:

• You know your LMP clearly: Start with LMP dating, then confirm at the first scan.
• Your cycles are irregular or your LMP is fuzzy: Put more weight on early ultrasound dating.
• You tracked ovulation with tests: Keep that date in your notes, but expect clinics to translate it into gestational weeks.
• You did IVF: Use transfer dating as your anchor.

The aim is one clean week count that stays steady across the rest of pregnancy. Chasing different app outputs tends to raise stress without improving clarity.

A Simple Checklist To Keep Your Dating Clean

Before your next appointment, this checklist can save back-and-forth on dates:

• Confirm the first day of your last normal period (not spotting)
• Write down average cycle length from the last few months
• List any ovulation test dates if you used them
• Bring your first scan report date and the gestational age listed
• Use one primary due date for planning once your clinic sets it

When you walk in with those details, your clinician can set dating cleanly and keep care timing consistent from that point on.