How far along am I? Why your test, app and ultrasound dates differ

Gestational age or conception age?

Doctors and midwives date a pregnancy from the first day of your last period. That count is gestational age, and the Merck Manual calls it “the universal standard” (Merck Manual).

Conception age, which the Merck Manual calls embryologic age, counts from fertilization instead. Gestational age starts about 2 weeks before conception, so conception age is typically 2 weeks less (Merck Manual). Oddly, you aren’t actually pregnant for the first week or two of the 40 (Mayo Clinic).

You can know your period date. Conception is harder to pin down, partly because sperm can live inside your body for up to 5 days (Cleveland Clinic).

A worked example

1. September 1, 2026: your period starts. The count starts at 0 weeks.
2. About September 15: ovulation and conception, in a 28-day cycle. Conception age starts here.
3. About September 29: your period is late. You’re 4 weeks pregnant, and the embryo is about 2 weeks old.
4. October 16: 45 days in. You’re 6 weeks and 3 days pregnant, but only about 4 weeks and 3 days past conception.
5. June 8, 2027: your due date. It’s 280 days, or 40 weeks, after September 1.

How to work out your weeks and due date

Two sums turn a period date into a due date:

• Add 280 days. By convention, says the American College of Obstetricians and Gynecologists (ACOG), the due date is 280 days after the first day of your last period (ACOG Committee Opinion 700).
• Or use Naegele’s rule. Add 7 days, subtract 3 months, then add a year (Cleveland Clinic).

Because months differ in length, Naegele’s rule can land up to 3 days after the 280-day count. For a period that started on May 4, 2026, it gives February 11, 2027, against February 8 by 280 days. That alone can make two calculators disagree.

How weeks are written

Weeks are counted as completed weeks plus days: 6+3, 6 3/7 and 6w3d all mean 6 weeks and 3 days. ACOG writes 13 6/7 weeks, and UK hospital guidance writes 11+2 weeks (ACOG, NHS Greater Glasgow and Clyde).

“6 weeks pregnant” covers 6+0 through 6+6, the 7 days of your seventh week. Official statistics count the same way: 280 to 286 completed days after your period started is 40 weeks (Eurostat).

If your cycle isn’t 28 days

The 280-day count assumes a regular 28-day cycle, with ovulation on day 14. ACOG notes that it doesn’t account for misremembered dates, irregular cycles or the variable timing of ovulation (ACOG Committee Opinion 700).

Cycle length mainly changes the days before ovulation. A 2019 analysis looked at 612,613 ovulatory cycles logged by 124,648 users of a fertility-tracking app, mostly in Sweden, the UK and the U.S. The app estimated the day of ovulation from body temperature readings. The days before ovulation averaged 19.5 in cycles of 31 to 35 days, against 12.4 in cycles of 21 to 24 (Bull and colleagues, npj Digital Medicine, 2019). So with longer cycles, conception tends to come later than day 14, and your pregnancy may be a few days younger than your period date says.

Not sure of the date? ACOG cites reports that only about half of women accurately recall their last period. If you’re unsure, it says dating should be based on ultrasound, ideally by 13 weeks and 6 days (ACOG Committee Opinion 700).

What pregnancy tests and apps are counting

A standard home test looks for hCG, a hormone the placenta makes in pregnancy. It tells you whether your level is raised, meaning you’re pregnant, not how far along you are (FDA). Home pregnancy test or ultrasound compares what a test and a scan can each tell you.

Tests with a weeks reading

Some digital tests also estimate when you conceived. One example is Clearblue’s Weeks Indicator test, sold in the UK and elsewhere. It adds “1-2”, “2-3” or “3+” to a “Pregnant” result. Those are weeks since conception, not weeks of pregnancy. Clearblue’s own table converts them, based on a 28-day cycle (Clearblue):

• 1-2: you conceived about 1 to 2 weeks ago, which your doctor or midwife would count as 3 to 4 weeks pregnant.
• 2-3: about 2 to 3 weeks ago, or 4 to 5 weeks pregnant.
• 3+: more than 3 weeks ago, or 5 weeks or more.

In short, add 2 weeks. The reading stops at “3+”, so past about 5 weeks it can’t tell you more.

Apps and online calculators

An app works from the date you give it and the rule it’s built on. If it counts from your period with either rule above, it carries the same 28-day assumption. Yet in the 2019 study above, only 13% of cycles were 28 days long, and the authors called the belief that ovulation consistently falls on day 14 “not correct.” Two apps can also disagree because:

• They start from different dates. A period date, a conception date and an IVF transfer each need a different sum: 280 days, 266 days, or 261 to 263 days (Cleveland Clinic).
• They label weeks differently. “6 weeks” and “week 7” can mean the same day.

Once your doctor or midwife gives you a due date, enter it in every app, so they all count from the same day.

IVF due dates

After IVF, conception timing is known, so no period date is needed. ACOG says the gestational age from assisted reproduction should set the due date, using the embryo’s age and the transfer date (ACOG Committee Opinion 700):

• Day-5 embryo: the due date is 261 days after the transfer.
• Day-3 embryo: the due date is 263 days after the transfer.

Counting 261 days from a day-5 transfer is the same as counting transfer day as 2 weeks and 5 days (19 days) of pregnancy. A day-5 transfer on October 1, 2026 gives a due date of June 19, 2027. Use your fertility clinic’s date, whatever an app says. What the clinic’s first scans check.

How an ultrasound dates a pregnancy

Early on, the scan measures the embryo or baby from the top of the head to the bottom of the rump: the crown-rump length. ACOG calls first-trimester ultrasound, up to 13 weeks and 6 days, “the most accurate method to establish or confirm gestational age.” In that window, it’s accurate to within 5 to 7 days either way. Later scans use head, belly and thigh-bone measurements, with wider margins. ACOG gives 7 to 10 days up to 21 6/7 weeks, 10 to 14 days up to 27 6/7, and 21 to 30 days after that (ACOG Committee Opinion 700).

When the due date changes

In the U.S., ACOG’s table lists the gaps between scan and period dates that support changing the due date, counting weeks from the period date. Smaller gaps can also justify a change, depending on how early the scan was and how reliable the period date is (ACOG):

These rules are for medical scans, and only your doctor or midwife can apply them to you. Late in pregnancy, a small measurement could mean a baby isn’t growing as expected, so ACOG calls decisions based on a third-trimester scan alone “especially problematic.”

ACOG also says a pregnancy without an ultrasound that confirms or revises the due date before 22 0/7 weeks “should be considered suboptimally dated.” In England, the NHS offers a scan at around 10 to 14 weeks, partly to work out your due date (NHS).

Once it’s set, it tends to stay

ACOG says later changes to the due date “should be reserved for rare circumstances.” With rare exception, a second-trimester scan shouldn’t change dates set by a first-trimester one, and your doctor or midwife decides what a later measurement means.

For the scan itself, read what to expect at your first pregnancy ultrasound; for what shows each week, see how early an ultrasound can see a pregnancy and when you can see the heartbeat.

Why your due date is an estimate

Few babies arrive on their due date. An Australian study used birth records for 18,708 women in one health district who went into labor on their own and had one live-born baby without a major birth defect, between 2007 and 2011. Only 5% gave birth on their estimated due date, whether that date came from a period or a scan. About 66% gave birth within 7 days either side (Khambalia and colleagues, International Journal of Gynecology & Obstetrics, 2013).

Some of that spread is natural. A U.S. study in North Carolina, from 1982 to 1985, followed 130 naturally conceived pregnancies and analyzed 125, timing ovulation from hormones in daily urine samples. The median time from ovulation to birth was 268 days (38 weeks and 2 days). Even with six preterm births left out, pregnancy lengths ranged over 37 days (Jukic and colleagues, Human Reproduction, 2013, ESHRE).

So ACOG and the Society for Maternal-Fetal Medicine label a window, not a day (SMFM, Merck Manual):

• Preterm: before 37 weeks.
• Early term: 37 0/7 to 38 6/7 weeks.
• Full term: 39 0/7 to 40 6/7 weeks.
• Late term: 41 0/7 to 41 6/7 weeks.
• Postterm: 42 0/7 weeks and beyond.

The date still matters. ACOG calls an accurate due date, set early in prenatal care, “among the most important results of evaluation and history taking” (ACOG). It times care, tests and growth checks.

When to tell your doctor or midwife about a date mismatch

Small differences between a test, an app and your period date are expected. These are worth a call:

• A scan showed less than your dates predict, at a medical or a keepsake scan: a smaller pregnancy, no heartbeat yet, or an empty-looking screen. It can simply mean it’s early, or that the dates are off. It can also be a sign of a problem, and only your doctor or midwife can tell which. Tell your doctor or midwife, even if you feel well. Measuring behind on your first ultrasound and No heartbeat on an early ultrasound explain what can happen next.
• You’re unsure of your period date, your cycles are irregular, or you’re on birth control. A period-date sum is less reliable then (Cleveland Clinic), so mention it at your first visit.
• Your first visit is late and you’ve had no dating scan. Ask whether you need one. Why your first prenatal appointment isn’t until 8 to 12 weeks explains the usual timing.

If you have pain or bleeding, don’t wait for a scan to sort out your dates.

Your dates and a first look in Glendale

Our week planner turns a due date into the weeks for each of our sessions: the Pregnancy Check in the first 10 weeks, Gender Confirmation from 13 and 4D Bonding from 16.

Type in a due date and the planner marks the sessions you could book now and the one that opens next. Whatever you enter stays on your device.

Our Pregnancy Check is a short, private 2D ultrasound at our studio in Glendale, Los Angeles County: a few minutes on a big screen, with 8 black-and-white prints to take home. We point out what you’re looking at and tell you what we could and couldn’t see. It’s $100, one price, with no referral or insurance.

Call (818) 243-7010 first and tell us the first day of your last period, and we’ll tell you whether it’s a good day to come. We can’t date a pregnancy or give a due date. If it’s too early to see, you come back for a second look, almost always at no charge. Same-day appointments are often available.

With nearly two decades of experience, Irina, our lead sonographer, has done more than 30,000 pregnancy ultrasounds. The FDA and the American Institute of Ultrasound in Medicine discourage scans done only for keepsakes; Safety, in plain words has their statements. See how the Pregnancy Check works.

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