“90% sure it’s a girl”: what a sonographer’s confidence means

What “90% sure” means, and what it doesn’t

When a sonographer says “90% sure it’s a girl,” the number is a judgment, not a measurement. It sums up how good the view was: the angle, how the baby was lying and how long there was to look. It doesn’t come from a study or a chart, so it can’t be read as a 9-in-10 chance for your baby.

What a scan detects is the baby’s sex, from the anatomy on the screen, and the call is made by looking. For the 20-week anomaly scan, a guideline from NHS Greater Glasgow and Clyde, a Scottish health board, asks sonographers to decide “under direct observation,” checking the area between the legs in three different views. A boy call needs a clear view of the penis and scrotum, and a girl call needs the parallel lines of the labia (NHS Greater Glasgow and Clyde). At 11 to 14 weeks, the studies below made their calls from a side view instead.

The guidance and studies in this article don’t grade confidence in percentages. They treat certainty as a line: either the sonographer is sure enough to make a call, or they say they can’t tell.

• A Scottish health board. Its sonographers share the sex only if they’re “able to determine fetal sex with certainty,” and otherwise record that they couldn’t. Parents are told it “is not always 100% accurate.”
• A U.S. hospital study. At Miami Valley Hospital in Dayton, Ohio, researchers measured the angle of the genital tubercle, the “nub,” on belly scans at 11 to 14 weeks. “At an intermediate angle of 10–30°, the sex was not assigned.” A sex was assigned in 534 of 683 cases (78%), and 94.4% of those calls matched a later scan in mid-pregnancy. The abstract also counts 567 babies, without explaining the gap (Schaefer and colleagues, 2023).

The calls that aren’t made

Counting the scans that got no call separately makes a big difference. In a 2014 study at a specialist private obstetric practice in the Illawarra region of New South Wales, Australia, seven sonographers scanned 640 babies across all three trimesters and predicted the sex where they could. They used a side view in the first trimester, and the answers were checked after birth (Kearin and colleagues, 2014).

At 11 to 14 weeks, 75% of scans gave a correct answer. Counting only the scans where a call was made, 91% were right. The abstract doesn’t say how many babies were in that group, or how many got no call.

Later scans show the same pattern. At an ultrasound practice in Victoria, Australia, 843 babies were scanned at 14 to 20 weeks, when their mothers had amniocentesis for genetic reasons. A prediction was recorded for 770 (93.3%), and 765 of those 770 (99.3%) were right (Meagher and Davison, 1996).

So a “can’t tell” isn’t a failed scan: each of the studies above had them. A hedge is a close cousin. It tells you the view fell short of what the sonographer needed to be sure. For the evidence week by week, see How accurate is a gender ultrasound at 13, 14, 15 and 16 weeks?

What raises or lowers a sonographer’s certainty

The week

Earlier calls are less certain. In the Illawarra practice, the “overall success rate” before 12 weeks was 54%, and every call made after 14 weeks, into the third trimester, was right. The abstract doesn’t give the size of either group, or say whether the 54% counts scans with no call. Its authors advise against calls before 12 weeks, as does the Australasian Sonographers Association (ASA) in a 2022 position statement (ASA, 2022). In the Dayton study, accuracy rose with each week band from 11 to 14 weeks.

The views

A sure call rests on something seen, for a girl as much as for a boy. In the Victoria series, at 14 to 20 weeks, the authors suggested combining a side (sagittal) view with a cross-section (transverse) or tangential view to cut errors. The Scottish guideline asks for all three at the 20-week scan. Can an ultrasound be wrong about the gender? explains what can mislead even a careful look.

The baby’s position and your body

The baby has a say. In England, the NHS warns that “if your baby is lying in an awkward position, it may be difficult or impossible to tell” (NHS). At the anatomy scan, too, Cleveland Clinic says seeing the genitals depends on the baby’s position (Cleveland Clinic). Body build matters as well. In a 2016 study of 400 pregnancies at 11 to 13+6 weeks at a Hong Kong hospital, a baby lying face down and obesity in the mother both made a call significantly less likely (Kong and colleagues, 2016).

Time and experience

A rushed look lowers certainty. In a 1996 study by London doctors, 472 women had the sex checked during their routine 20-week anomaly scan, with no extra time allowed for it. For 50 of them (10.6%), the sex couldn’t be told in the time given, and 14 of the calls made (3.3%) were wrong (Harrington and colleagues, 1996). Experience counts too: in the Illawarra study, success rates differed between individual sonographers.

Why “come back” beats a guess

A “not sure” costs you a few weeks. A wrong guess can cost more: clothes to return, a second reveal, news to take back. The ASA warns that an incorrect call “has potential for psychological implications.”

Guidance for sonographers backs the wait. For first-trimester scans, the ASA says sonographers should explain that “there will be an opportunity for assessing the sex with more accuracy at a later gestational age.” The NHS Greater Glasgow and Clyde guideline says that if the genitals can’t be seen clearly in the time allowed, the 20-week scan “should not be prolonged or repeated” to find the sex. Both accept “can’t tell” as an answer, though the Scottish guideline means no second try for the sex there.

A later look is a surer one, as the studies above found. Cleveland Clinic goes further: in its 2021 article, obstetrician Justin Lappen says that “before 16 weeks or so,” ultrasound isn’t a reliable way to determine the sex (Cleveland Clinic, 2021).

The obvious later look is the medical anatomy scan, at about 18 to 22 weeks in the U.S. (Cleveland Clinic) and usually 18 to 21 weeks in England (NHS). It checks your baby’s growth and well-being (Cleveland Clinic, 2021); the sex isn’t the point, so ask. The U.S. practice guideline lists the external genitalia with a note: “If medically indicated or the patient wants to know” (ACR–ACOG–AIUM–SMFM–SRU). In England, finding out isn’t part of the national screening program, and whether you can depends on your hospital’s policy (NHS).

Another elective scan is the other route, where the law allows one. It’s still a scan done for a keepsake, and the FDA and the American Institute of Ultrasound in Medicine (AIUM) discourage those (FDA, AIUM). Safety, in plain words has their statements. For every other way to find out, from blood tests to birth, see When can you find out your baby’s gender?

What to ask in the room

You can ask how sure the sonographer is, and why. These questions turn a percentage into a plan:

1. “How sure are you, and what’s holding you back?” The answer tells you whether it was the week, the position or the view.
2. “What did you see?” Ask them to point it out on the screen.
3. “Did you see it from more than one angle?” From about 14 weeks, a side view backed by a second view is stronger than one frame.
4. “Was the baby lying well?” An awkward position can hide the area completely.
5. “Would a later look help, and when?” Write down the week they suggest.
6. “If it goes in an envelope, can you tell me how sure you were?” Then nobody throws a party on a guess.

At a medical scan, speak up early. In England, the NHS advises telling the sonographer “at the start of the scan” that you’d like to know. It also notes: “Some hospitals have a policy of not telling patients the sex of their baby.”

Hedged call? When to buy pink or blue

Treat a hedged call as a best guess, not a final answer, until someone has had a clear look. You can still get ready:

• Buy neutral basics now. Plain sleepers, swaddles, burp cloths, a car seat and a crib work for any baby.
• Keep receipts and tags on anything pink or blue until you have a confident call, which may not come until the anatomy scan. Check the return window before you buy.
• Ask for gift receipts if family can’t wait to shop.
• Say “we think” rather than “it’s” when you share the news.
• Keep both name lists going. Boy or girl? What changes when you find out early covers planning, names and why waiting is fine too.
• Hold the reveal until someone is sure. How to plan a gender reveal party, step by step has the rest.

If a blood test has already reported the sex and a scan disagrees, tell your doctor or midwife. There are several possible explanations, a few of them medical, and only they can look into it (Dhamankar and colleagues, 2020). The article on whether an ultrasound can be wrong about the gender covers what to do when results clash.

What not to read into a call

A hedged call invites tie-breakers. These don’t break the tie:

• The heart rate. “The fetal heart rate does not predict the sex of the baby,” says Justin Lappen, an obstetrician at Cleveland Clinic. He adds that studies have found no difference between male and female heart rates.
• The “nub” on its own. In research, the nub’s angle is measured on a true side view, and in some studies, in-between angles get no call. Before 12 weeks, the ASA discourages calls at all. A screenshot judged by strangers online is not that measurement.
• A single print. The sonographer judged a moving picture. A print is one frame of it.
• Bump shape, cravings and other tales. The old wives’ tales quiz puts them to the test, for fun.

Keeping it a surprise

If you don’t want to know, or want the answer sealed for someone else, say so before the scan starts. The NHS Greater Glasgow and Clyde guideline asks sonographers to “establish at the start of the examination whether the patient wishes to know the sex of the baby.” Ask, too, whether the screen can be turned away while they look.

Envelopes aren’t universal, so ask ahead. Under the same guideline, “No envelopes will be provided”: the sonographer circles the answer on a tear-off slip, which is folded and handed to you.

A sealed answer is only as good as the view behind it. Before it’s sealed, ask the sonographer to tell you if they couldn’t be sure. A card that says “come back” beats a guess opened in front of everyone. Then set the reveal date after a confident call.

How we handle a maybe, in Glendale

At our studio in Glendale, Los Angeles County, when we can’t be sure, we say so, and you come back for a free re-check. If we can’t tell with confidence, we don’t guess. No ultrasound can promise 100%. If your baby is being shy, we take longer.

Our Gender Confirmation is a 2D scan, the classic black-and-white picture. We book from 13 weeks, and we’re surest from 15. If you can wait until 15, do: we’re more confident, and a second look is less likely. If what we see is different from what you’ve been told, we tell you what we see, and how sure we are. What a blood test result means is a question for your doctor or midwife.

Tell us it’s a secret before you lie down. The screens stay off, we find the answer, and it goes into an envelope nobody can see through. The envelope is subject to availability, so ask for it when you book. It’s a keepsake, not a medical exam: it doesn’t replace the anatomy scan or any appointment with your doctor or midwife. Finding out with us, checking what you’ve been told, prices.

Counting down to a clearer look? Enter your due date and the planner shows which of our sessions are open to you now and which come next. It all stays on your device.

Questions