Measuring behind on your first ultrasound: what it can mean

What “measuring behind” means

At an early dating scan, the sonographer measures the embryo and converts its length into an age, in weeks and days. “Measuring behind” means that age is younger than the weeks counted from the first day of your last period. If your period dates say 7 weeks and 2 days and the scan says 6 weeks and 3 days, the pregnancy is measuring 6 days behind.

The period count is a calendar estimate, built on an assumption about when you ovulated. The scan measures the embryo itself. In the first trimester, the American College of Obstetricians and Gynecologists (ACOG) calls an ultrasound measurement “the most accurate method to establish or confirm gestational age” (ACOG Committee Opinion 700).

What a gap means for your pregnancy depends on the rest of the scan and on how sure your dates are. Only your doctor or midwife can weigh that.

How a measurement becomes a date

The measurement is the crown-rump length: the embryo’s length in a straight line from the top of its head to its bottom. ACOG asks for it to be taken in a true side-on view, and for dating to use the average of three separate measurements when possible (ACOG). The length is then converted into an age. For typical sizes week by week, see How early can an ultrasound see a pregnancy?, and for the scan itself, What to expect at your first pregnancy ultrasound.

Up to 13 weeks and 6 days, ACOG puts the accuracy of crown-rump dating at plus or minus 5 to 7 days. A few days either way is within that margin.

The period date is an estimate too. It assumes a 28-day cycle with ovulation on day 14. ACOG notes that this doesn’t allow for a misremembered date, irregular cycles, or ovulation that comes earlier or later. It also cites reports that only about half of women recall their last period accurately.

In a 2004 trial that ACOG cites, 40% of the women assigned a first-trimester scan had their due date adjusted. Their scan and period dates had differed by more than 5 days. How far along am I? sets out the full dating rules.

Reasons a pregnancy can measure behind

Each of these can make a pregnancy measure younger than its period dates.

Later ovulation

In a study by researchers in Cambridge, England, 143 women trying to conceive tracked ovulation with home urine tests. Ovulation that led to a pregnancy came “as early as day 11 in the cycle and as late as day 39.” The median was day 16 (Mahendru and colleagues, Archives of Gynecology and Obstetrics, 2016). Ovulating a week late makes a pregnancy about a week younger than the period count says.

Later implantation

Implantation timing varies too. In a U.S. government study run from 1982 to 1986, 221 women trying to conceive collected daily urine samples (NIEHS). In the 141 pregnancies that lasted at least 6 weeks, implantation, timed by the first trace of pregnancy hormone in the urine, came 6 to 12 days after ovulation. In 118 of them (84%), it came on day 8, 9 or 10 (Wilcox and colleagues, New England Journal of Medicine, 1999). The same study found that later implantation went with more losses before 6 weeks.

Does that timing show up on a scan? In an earlier paper, the Cambridge team compared 71 of their ongoing pregnancies. Later implantation went with a smaller crown-rump length at 10 to 14 weeks, and earlier implantation with a larger one. They concluded that implantation timing “largely explains” the variation in first-trimester dating (Mahendru and colleagues, Ultrasound in Obstetrics & Gynecology, 2012). The study was small, and its finding is about size at 10 to 14 weeks, not at 6 or 7.

Long or irregular cycles

Cleveland Clinic notes that “most people ovulate 14 days before their periods begin,” and that a “normal” cycle runs anywhere from 21 to 35 days (Cleveland Clinic). On a 35-day cycle, that puts ovulation around day 21, a week later than the calendar assumes. If your cycles vary, the guess is harder still: ovulation timing “varies from person to person and even from cycle to cycle.”

An unsure period date

A start date that’s a few days off moves every week of the count. ACOG says that when someone is unsure of their last period, dating should rest on the ultrasound, with the earliest crown-rump measurement counted as the most reliable. If you’re not sure of your date, say so at the scan.

Ordinary differences in measuring

At a fetal medicine referral center, two examiners both took measurements in 44 pregnancies at 6 to 9 weeks. Their 95% limits of agreement for crown-rump length were plus or minus 14.64%. In plain terms, in 95 cases out of 100, their results for the same embryo would be expected to differ by less than 15% (Pexsters and colleagues, Ultrasound in Obstetrics & Gynecology, 2011). The authors called that reasonable for dating, but urged care, when diagnosing miscarriage, “for values close to any decision boundary.”

Any of these can explain a pregnancy that measures behind. Measuring behind 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, especially if someone else did the scan.

New due date, or a closer look?

ACOG gives doctors guidelines for when a first-trimester scan should change the date. Up to 8 weeks and 6 days, a difference of more than 5 days between the scan and your period dates supports changing the due date. From 9 weeks to 13 weeks and 6 days, a difference of more than 7 days does. Smaller gaps can also justify a change, depending on how early the scan was and how reliable your period date is (ACOG). Once the date is set, ACOG says later changes should be “reserved for rare circumstances.” How far along am I? covers the rest.

In England, the NHS offers the first routine scan at 11 to 14 weeks, and estimates the due date then from the baby’s measurements (NHS). NICE guidance for doctors in England also says not to use the period date alone to decide whether a heartbeat should be visible (NICE).

A new date isn’t the only possible response. A review for family doctors lists findings that are “suspicious for early pregnancy loss” but not enough to diagnose it. When one turns up, it says to repeat the scan in 7 to 10 days (AAFP). The list includes:

• A small embryo with no heartbeat. At or above a set size, the review lists no heartbeat as diagnostic of loss rather than suspicious. These are rules doctors apply to medical scans, and only a doctor can apply them. No heartbeat on an early ultrasound explains them.
• A sac that’s small for the size of the embryo.
• A slow heart rate. When can you see the heartbeat on an ultrasound? has typical rates by week.

Doctors also compare one scan with the next. A study at four London hospitals recruited 1,060 pregnancies of uncertain viability: an empty-looking sac, or a small embryo with no heartbeat yet. Scans were repeated 7 to 14 days later. Among the 359 that still showed a sac on the second scan, embryos in pregnancies that continued grew faster between scans, on average, than those that didn’t. Even so, the authors said their data suggest that diagnosing miscarriage from growth is “potentially unsafe” (Abdallah and colleagues, Ultrasound in Obstetrics & Gynecology, 2011).

What research says about a small first measurement

Some research links a small first measurement to later loss. A 2008 study at a London hospital’s early pregnancy unit reported on 292 women whose pregnancies looked viable at a first scan between 5 and 10 weeks. All had conceived without treatment, were sure of their last period and had regular cycles. In the 41 pregnancies that later miscarried, every embryo had measured below the average size for its dates. In 25 of them, it was far below, by at least two standard deviations (Mukri and colleagues, BJOG, 2008).

That finding has limits. The pregnancies that continued had also measured below the average for their dates, on average, though by less. The published summary compares groups; it doesn’t give the odds for any one pregnancy. And it left out anyone unsure of their dates or with irregular cycles. Only your doctor or midwife can say what yours means.

What happens next

Ask your doctor or midwife which path you’re on: a new due date, a second scan, or both.

If they want to look again, the AAFP review suggests a repeat scan at 7 to 10 days for suspicious findings. The London study rescanned at 7 to 14 days. The second scan lets your doctor or midwife compare growth, look again for a heartbeat if there wasn’t one, and check the sac against the embryo.

NICE, which advises the NHS in England, says women should be told what to expect while waiting for a repeat scan. It adds that waiting “has no detrimental effects on the outcome of the pregnancy” (NICE NG126). Before you leave, ask who to call if anything changes, day or night.

If you conceived with IVF

With IVF, your dates come from the lab, not the calendar. ACOG says the due date for an IVF pregnancy should be set “using the age of the embryo and the date of transfer” (ACOG). So the scan doesn’t reset it, and late ovulation can’t explain a small measurement. If an IVF pregnancy measures smaller than expected, it can be a sign of a problem, and only your doctor or midwife can tell what it means. Tell your doctor or midwife, even if you feel well, and ask your fertility clinic too. What the clinic’s early scans check.

Waiting for the next scan

A week or two can feel very long. A few things can make it easier to carry.

• Write your questions down. What else did the scan show? What will they compare next time? What would change the plan?
• Go easy on forums. Threads about measuring behind hold stories that went both ways, and none of them is yours.
• Lean on someone. Tell one person you trust, if that feels right.
• Keep your other appointments. That includes your first prenatal visit: why it isn’t until 8 to 12 weeks, and what to do until then.

If the worry is taking over, tell your doctor or midwife. In the U.S., the National Maternal Mental Health Hotline, 1-833-TLC-MAMA (1-833-852-6262), is free, confidential and open 24/7 in English and Spanish, by call or text (HRSA). Postpartum Support International’s HelpLine is 1-800-944-4773: you leave a confidential message, and a trained volunteer calls or texts you back. PSI also runs online support groups, including for pregnancy loss (PSI). In the UK, the Tommy’s Helpline, run by midwives, is free on 0800 0147 800, 9am to 5pm on weekdays (Tommy’s).

Is an early reassurance scan worth it? has more on scan anxiety, and a section for pregnancy after a loss.

Can a keepsake scan tell you if you’re measuring behind?

No. An elective scan, ours included, can’t date a pregnancy or give a due date, so it can’t tell you whether yours is measuring behind, or what that would mean. If you’re waiting for a follow-up scan, that’s the scan your doctor or midwife will use to answer it. Tell your doctor or midwife about any worry, even between appointments.

What our Pregnancy Check offers is simpler. It’s a short, private 2D ultrasound in the first 10 weeks, at our studio in Glendale, Los Angeles County. You watch on a big screen, we point out what you’re looking at, and we tell you what we could and couldn’t see. You take home 8 black-and-white prints. It’s $100, with no referral and no insurance. Before you book, call (818) 243-7010 and tell us the first day of your last period, so we can tell you whether it’s a good day to come.

The FDA and the American Institute of Ultrasound in Medicine discourage scans done only for keepsakes, and Safety, in plain words has their statements. See what a Pregnancy Check includes.

Type in your due date, the new one if a scan changed it, to see which of our sessions you could book now and which open next. Your entry stays on your device.

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