How Do You Figure Out How Pregnant You Are? | Week Count Fix

Pregnancy dating sounds like it should be simple. Pick a date, count weeks, done. Then real life shows up: cycles vary, ovulation shifts, tests turn positive on different days, and apps disagree by a week.

This article walks you through the steps clinics use to label “how far along” you are, then shows what to do when dates don’t match.

What pregnancy age numbers mean

When people say “I’m 10 weeks,” they usually mean gestational age. Gestational age is counted from the first day of your last menstrual period (often shortened to LMP), not the day you conceived. That detail explains why the week count can feel ahead of what you think happened.

Two other terms show up a lot:

• Estimated due date (EDD): A target date near 40 weeks of gestational age. It’s a planning anchor, not a promise.
• Fetal age (sometimes called embryonic or conception age): A rough count from conception. It usually runs about two weeks behind gestational age in people with a cycle near 28 days.

Clinics stick with gestational age because it lines up with test windows and growth charts. Once the date is set, stick with it.

How Do You Figure Out How Pregnant You Are? Start with your last period

If you know the first day of your last period, you can estimate how pregnant you are in minutes. This method assumes you ovulated around the middle of your cycle, so it fits best when your cycle is steady.

Step 1: Find the first day of your last menstrual period

Check your calendar, period tracker, or notes. The date you want is day 1 of bleeding, not the last day. If you’re unsure, pick the closest reasonable date and mark it as a guess so you don’t treat it like a fixed fact later.

Step 2: Count weeks and days from that date to today

Count the number of days between your LMP date and today. Then divide by 7 to get weeks, with the remainder as extra days. If you don’t want to do the math, the NHS due date calculator uses the same baseline approach.

Step 3: Estimate your due date with the 40-week rule

A common estimate adds 280 days (40 weeks) to the first day of the last period. Many clinicians describe the same idea as adding 7 days, then counting back 3 months, then adding 1 year. The shortcut lands you in the same neighborhood.

Step 4: Adjust if your cycle isn’t 28 days

If your cycle is usually longer or shorter than 28 days, your ovulation day may shift, and the LMP estimate can drift. A simple adjustment is to add the difference between your cycle length and 28 days. A 32-day cycle adds 4 days; a 25-day cycle subtracts 3 days.

This is still an estimate. Ovulation can move even in people with steady cycles, and a stressful month or illness can shift timing.

Figuring out how pregnant you are when cycles aren’t regular

If your cycle changes a lot month to month, the LMP method can be misleading. In that case, use any firm dates you do have, then plan to confirm the timeline with imaging.

Use your first positive test as a reality check

A urine pregnancy test can’t date a pregnancy by itself, but it can tell you if an LMP-based count feels impossible. A positive test usually happens after implantation and rising hCG, so a result that comes “too early” often means your LMP date is wrong or your cycle ran longer than usual.

If you tracked ovulation, you can estimate from that date

If you used ovulation strips, basal body temperature, or a fertility monitor, you can estimate conception timing. A rough rule: gestational age is about 2 weeks plus the number of weeks since ovulation. So, 6 weeks since ovulation maps to about 8 weeks gestational age.

Ovulation tracking can narrow the window when your last period is a blur.

If you did IVF, use the transfer date

With IVF, clinics often date the pregnancy from embryo age and transfer day. Your fertility clinic can tell you your gestational age on transfer day and your due date. That date tends to be cleaner than LMP dating because the timing is documented.

When an ultrasound sets the official timeline

Many people get a dating ultrasound in early pregnancy, especially if cycles vary, bleeding was unusual, or the LMP date is uncertain. In the first trimester, ultrasound measurement is widely used to establish or confirm gestational age. The American College of Obstetricians and Gynecologists notes that first-trimester ultrasound (up to 13 weeks 6 days) is the most accurate way to establish or confirm gestational age in routine care. ACOG’s methods for estimating the due date explains how clinicians pick and document the estimated due date.

What the scan measures early on

Early scans often measure crown-rump length, which is the length from the top of the embryo or fetus to the bottom of the torso. Later scans use other measurements like head size and femur length. The earlier scan tends to be tighter for dating because growth variation is smaller in the first trimester.

Why your due date may change after the scan

If your ultrasound estimate differs from your LMP estimate by enough days, the due date may be updated. That can feel odd, but the goal is a consistent clock for the rest of pregnancy. Once set, clinicians usually keep that date steady unless there’s a strong reason to change it.

What ultrasound can and can’t do

Ultrasound dating estimates gestational age; it can’t pinpoint the exact day of conception. It’s still the best tool when LMP is uncertain. The World Health Organization summarizes how ultrasound fits into routine antenatal care and why access to quality imaging matters for pregnancy management. WHO’s ultrasound technical brief for pregnancy before 24 weeks gives the policy view and the role of imaging in care.

After you have an established due date, week counts become easier. You can anchor each appointment and test window to that one date.

Dating method cheat sheet for real-life scenarios

Match your situation to the method below, then write down the dates you used.

Dating method	What you need	When it fits best
LMP + 40-week count	First day of last period	Cycles are steady and you’re sure of the date
LMP adjusted by cycle length	LMP date + usual cycle length	Cycles are steady but not near 28 days
Ovulation tracking estimate	Ovulation day from strips, temperature, or monitor	Cycles vary yet you tracked ovulation closely
IVF transfer dating	Embryo age + transfer date	Pregnancy conceived through IVF
First-trimester ultrasound dating	Early scan measurement	LMP uncertain, irregular cycles, or mixed signals
Second-trimester ultrasound estimate	Mid-pregnancy scan measurement	No early scan was done and dates are unclear
Reverse count from a confirmed due date	Clinic-provided EDD	You have an official due date and want “weeks today”
Unknown dates start point	Scan + symptom timeline + exam	LMP unknown and early care started late

How to calculate “how far along” once you have a due date

After an ultrasound or clinic visit gives you an estimated due date, you can work backward to find how pregnant you are on any date. Here’s the simple method:

1. Count the days from today to your due date.
2. Subtract that number from 280 days (40 weeks).
3. Convert the result to weeks and days.

The due date is the anchor; you’re counting back from it.

Why “how pregnant you are” affects timing of tests and visits

Dating isn’t trivia. Many screenings are offered inside a narrow week range. If your week count is off, you can show up too early for one test or too late for another. The CDC outlines common timing windows for first-trimester screening, including the 11–13 week range. CDC page on screening for birth defects summarizes how ultrasound and blood tests are used in that first-trimester window.

Visits follow a schedule too. A due date helps you plan booking visits, labs, vaccines, and later monitoring. Ask your clinic what they schedule by week in your area.

Table of week ranges you can use for planning

Use the table below as a writing aid. It keeps your notes organized so you can answer the “how many weeks?” question fast at any appointment or form.

Week range	What dating drives	What to record
Weeks 4–6	Early confirmation timeline	LMP date, first positive test date
Weeks 7–10	Early scan timing in some clinics	Any bleeding dates, pain notes, scan date if done
Weeks 10–14	Dating scan window in many systems	Scan result, confirmed due date, updated week count
Weeks 11–13	Some screening windows	Lab dates, ultrasound dates, results summary
Weeks 18–22	Anatomy scan scheduling	Appointment date, notes you want answered
Weeks 24–28	Common lab timing blocks	Lab appointment dates, results to store
Weeks 36–40	Late-pregnancy visit cadence	Weekly visit dates, birth plan notes, ride plan

What to do when your dates don’t line up

If your LMP estimate and your scan estimate disagree, skip the “average.” Clinics pick one primary due date so the clinic uses the same clock. If you feel confused, ask the clinic to write the gestational age and due date on a printed visit summary or in your portal message.

If you have bleeding, one-sided pain, fainting, or fever in early pregnancy, get medical care right away. Those symptoms need prompt attention and aren’t something to solve with a calendar.

A simple way to keep your dates straight

Use one note on your phone with these four lines:

• LMP first day: ____
• First positive test: ____
• Confirmed due date (if set): ____
• Gestational age today: ____ weeks ____ days

That tiny record stops a lot of confusion. You can update the last line each week, and you’ll always have the dates a clinic or form asks for.