ISUOG Obstetric Ultrasound Guidelines What They Actually Require

The ISUOG guidelines set out what a proper obstetric scan must contain. ISUOG is the international body for ultrasound in pregnancy, and its practice guidelines are the standard clinics scan to the world over. The guidelines name the scans a pregnancy should have, the week each falls in, and the findings each must establish. They set the floor a scan must clear to count as complete.

The scans a pregnancy is asked to have

The guidelines ask for a small number of scans, each at its own point in the pregnancy. The earliest dates the pregnancy and counts the babies inside. By the midpoint, the scan turns to the baby’s body, read part by part. A scan late on, where it is offered, checks the growing. The table sets out the three. Some guidelines ask for two scans, some for more, the number set by a country and its resources, two the floor in many places. The mid scan is the one no guideline drops, its survey too valuable to skip. A scan booked at the right week reads what it is meant to, a dating scan in the first weeks and an anatomy scan near twenty. The early scan is offered to every pregnancy, the dating alone reason enough to scan. The early scan now takes a first look at the baby’s body too, the skull, the wall of the belly, the bladder and the limbs read by twelve weeks, a grave defect able to show this early. The guidelines are revised as the evidence grows, the first-trimester scan asked to do more now than a decade ago.

The early scan settles three things before any anatomy, whether the pregnancy lives, where it sits, and how many there are. A heartbeat seen is the proof of life, the thing a mother waits longest for. An early scan that finds no heartbeat in a baby of a size that should have one names a loss, the hardest news a scan carries, the call made only after a small baby is read again. The crown-rump length is the baby measured from the crown to the base of the spine, a straight line on a baby curled in the womb, the guideline setting the plane and catching the baby at its straightest. Two babies are read for whether they share a placenta, the membrane between them traced and counted, a shared placenta the harder one to carry. The membrane is read best early, its layers easy to count before the womb fills, a count left late far harder to make. Twins that share a placenta are scanned more often, every two weeks from the middle of pregnancy, the guideline setting the closer watch for the risk they carry. A shared blood supply can tip out of balance between them, a danger those scans are set to catch.

The first-trimester scan is the one that fixes the dates. A measurement of the baby end to end, the crown-rump length, dates the pregnancy more tightly than any later one. A pregnancy dated from the last period alone can run days or weeks out, a woman’s cycle the wobble in it, the scan dating from the baby itself and the guideline trusting the measure over the memory. The guideline names a window for each scan, a scan taken outside it of less use for the questions it was meant to answer. The guideline ranks the ways to date, the crown-rump length first, the head measure next where the first scan was missed, the last period last of all. A mother is told before the anatomy scan where the search falls short, the consent the guideline asks for, a scan a search the guideline calls one. The dates set here hold for the rest of the pregnancy, every later reading judged against them. The early scan offers a screen for the common chromosome problems, a measure of the fluid at the back of the baby’s neck read alongside the mother’s blood. The fluid is measured in a set plane, a thicker reading the one that raises the odds, the screen giving a risk figure a firmer test then settles. The guideline fixes the dates once and does not move them, a pregnancy redated at every scan losing the one steady mark a reader judges growth against. The growth scan late on reads the baby’s size against the weeks, the placenta and the fluid alongside, a baby falling off its line the finding it looks for and a small baby on a failing placenta the one it must not miss.

The mid-trimester scan is the one the guidelines spell out at length. It carries the anatomy survey, the part-by-part look for malformation. It is the scan a pregnancy turns on, the one chance to read the baby’s body in full before birth. A pregnancy that misses it goes to birth with the baby’s body unread, the gravest defects a surprise in the room.

What the mid scan must show

The guidelines hand the scanner a checklist of structures, each to be seen and judged normal. The list runs from the head down to the feet, an order a scanner keeps to so nothing is skipped. The guideline draws a line between the routine survey every baby gets and the detailed scan a specialist runs on a baby at risk. The routine list is the floor, the structures every scan must clear. The survey is set at eighteen to twenty-two weeks for a reason, the baby large enough by then to see in full with time still left to act on what shows. The guideline asks the survey worked the same way every time, a set order that leaves no structure to memory, a scanner who follows it on every baby building the habit that catches the rare.

The head is read first, the skull whole and the two halves even, the brain at two set planes, one across the ventricles and the cavum, one across the cerebellum behind. The face follows, the lips closed across the front, the profile and the eyes in place, a cleft the malformation the view is set to catch. The spine is run from the neck to the tailbone in two views, along its length and across, the skin over it unbroken. The chest carries the heart, and the heart takes the longest of all, built on the four-chamber view that catches the gravest defects, the great vessels followed out and crossing and the arch where it can be had. The lungs sit even on either side. The stomach sits low on the left and the heart points the same way, the two a check that the organs lie where they should. The kidneys are found either side of the spine, the bladder a pocket of urine between them, a bladder that fills and empties the proof the kidneys work. The cord joins the wall where it should, three vessels in it the normal count, the wall itself closed over the gut. The limbs are counted and run from the hip and the shoulder to the foot and the hand, the long bones measured. Each structure has its named plane in the guidelines, a view the scanner captures and stores. The guideline lists the views by name, a scanner working down them so the report can show each one was had. The survey is the full list on every baby, low-risk and high-risk alike. The neck and the back of the head are read for swelling, a thickened fold a soft pointer the guideline asks be measured. The scan records where the placenta lies and how much fluid surrounds the baby, both named in the guideline, a low placenta read again later and a low or a high fluid a finding to chase.

The guidelines set what counts as a complete view of each part. A heart seen only in the four-chamber view is a heart half-checked, the outflow tracts asked for as well. A spine seen only along its length misses a gap a cross view would catch. The survey is surest on the gross defects, a missing limb or an open spine read with near certainty. A hole in the heart’s wall, a narrowing that shows only after birth, these can slip past it, the guideline plain about the reach it has. A soft marker is a finding that leans the odds without proving anything, a bright spot in the heart or a slightly full kidney, the guideline naming which to report and which to let pass and a marker alone rarely the cause for the worry it brings. Some guidelines add a flow reading at the anatomy scan, the mother’s arteries to the womb read for the risk of pre-eclampsia, a high resistance there a pregnancy to watch. The output of the report is a rate, the share of the major defects a service catches, audited against what is later found at birth. A service that scans to the guideline measures itself this way and looks hard at what it misses. The neck of the womb is measured at the anatomy scan in many services, a short one the warning of an early labour, the guideline adding the measure where the screen is run.

The scanner stores an image of each required view. The picture is the proof the structure was seen, a record another reader can open and check. A scan with no image of the heart’s outflow has not met the standard, whatever the scanner saw on the day.

The survey takes the time the guidelines allow for, a full anatomy scan rarely a quick one. A scanner rushed through it is a scanner who misses. The survey catches a good share of the malformations there are to catch, some too small or too subtle to see, some that declare themselves only later in the pregnancy. The image a guideline needs can be hard to win on a heavy mother or a baby lying awkwardly, the same on any machine. A scanner returns another day for the views a position hid, the survey finished over two visits where one will not do.

The measurements that date and size

The guidelines ask for a set of measurements at the mid scan, the biometry. Four are standard, the head across and around, the belly around, the thigh bone along its length. Together they size the baby and date a pregnancy whose dates are not yet fixed. The head is taken two ways, across between the bones and around the whole, the round the steadier of the two when the head’s shape varies, the guideline leaning on the circumference for dating late. The head is measured in the plane that shows the cavum and the thalami, the calipers set on the bone. The belly is measured where the stomach and the vein meet, a round taken at the skin. The thigh is measured along the shaft alone, the soft ends left out. The charts run a line for the average and lines above and below it for the spread, a baby read for where it sits among them, the one below the lowest line flagged for a closer look. A baby measured well off its dates sends a reader back to the dating, a gap of more than a week or so the trigger to look again.

A measurement taken in the wrong plane reads wrong, the guideline tying each number to its view. Each measurement lands on a chart for the weeks, the baby read at its centile, the line it tracks scan to scan more telling than the place on it. A baby holding its own centile is a baby growing as it should. A single measurement places a baby on a chart, the trend across two or more drawing its growth, the guideline leaning on the trend over any one reading. Dating is tightest at the first scan, a baby’s size a surer guide to its age the smaller it still is.

The four measurements feed an estimate of the baby’s weight. The estimate carries an error of its own, a weight read within a tenth or so of the true figure. A reader holds it as a guide, the trend across scans steadier than any single weight. The guideline holds the figure as a flag for the small or the large baby, a number read with its error in mind. The belly’s round is the measure that catches a baby going short, the first to fall when a placenta fails, the guideline weighting it heavily in the read of growth. A baby measured large for its dates is read for the mother’s sugar, the guideline reading the large baby as carefully as the small. The four measurements run through a formula a machine works in a second. The guideline names the formula it expects, the weight read the same way from clinic to clinic so two readers land on one figure. The four measurements are taken the same way the world over, the guideline the reason a scan in one country reads in another. A baby estimated small or large shapes the plan for the birth, the figure read into how and when the baby comes, a guide a clinician weighs with its error in mind. A tape measure of the belly stands in where no scan is to hand, the guideline holding the scan above the tape and the number far tighter.

The dating measurement is the one that counts for the longest. A crown-rump length in the first trimester dates a pregnancy within days, the tightest dating there is. The guideline takes the dates from that earliest scan and holds them for the rest. A pregnancy sure of its dates is a pregnancy whose every later reading means something, the chart read against a date a reader can trust.

Who scans and what is kept

The guidelines ask for a trained hand and a machine equal to the task. A scanner learns the survey over hundreds of scans, the eye for the normal built before the eye for the rare, a checklist of little use to a hand that cannot find the views. A doubtful finding is taken to a second reader, the guideline building review into the harder calls. The neck measure that screens for chromosomes is held to its own standard, a scanner accredited to take it and audited on the numbers, a measure taken loosely throwing the screen off. The guideline sets a pace for a full survey, rarely done well in a hurry, a service that books too little time per scan missing on the hardest views. The output is held low over a baby, the beam no stronger and no longer than the reading needs. The thermal and the mechanical readings sit on the screen for the scanner to watch, a long stare at one spot the thing the guideline warns against. The output is held lowest in the first weeks, the early baby the more sensitive to the beam, a scanner keeping the time short and the power low over a baby just formed. The safety of the baby sits above the picture in the order of things. The guideline asks a machine that resolves the structures and measures true, an old or a poor machine failing the baby as surely as an untrained hand.

The images and the measurements are stored to the record. The store is the proof the scan was done to standard, a set another clinician can open and read. A scan that keeps nothing leaves no way to check it was complete. The guideline is written to be audited, a service able to show the rate it works to, a standard no one checks a standard in name alone. The scan ends in a report, the findings set down in plain words a clinician and a mother can both read. A finding is described, measured, and placed, the record showing what was seen and where. The report names what was seen, the parts a position hid set down for a return visit. A mother is told in plain words, a soft marker that often means nothing explained with care. A scan can turn up a finding no one looked for, a cyst on an ovary or a stone in a kidney, the mother’s own, the guideline asking a scanner to note what crosses the view.

What a normal scan does not rule out

The guideline sets a floor for a scan to clear, the health of the baby a larger question than any scan answers.

Meeting the standard on a handheld

A handheld can meet the guideline where its image is good enough. The standard asks what a scan shows, a question the image answers whatever the machine that draws it. A pocket probe that resolves the four-chamber heart and the brain’s planes can work the same checklist.

The early scan and the growth scan ask only the dating, the biometry, the fluid and the placenta, all within a handheld’s reach. The anatomy survey is the harder bar, the heart and the brain the views that test a probe, a probe meeting it where it draws those crisply. A clinic builds up to the survey when its scanners and its kit are ready. A handheld stores the views the guideline asks for, a clip a specialist can read from afar, a scanner who reaches the limit of a hand sending the images on.

A clinic that scans to the guideline on a handheld brings the standard to where the carts cannot go. The guideline does not lower for the setting, the same checklist asked wherever the baby is scanned. A handheld turns the growth scan from a hospital trip into a bedside check, a small baby followed week to week where the mother lives. A midwife trained to the guideline runs the dating and the growth scans on a handheld, the full anatomy survey kept for a specialist visit, the standard met across the two of them. A handheld in a clinic does the first look, a scan that finds a flag sending the mother on to a specialist, the guideline letting that first look happen close to home. A small baby found on a handheld is followed on it, the growth read week to week and the worst sent on for the cord Doppler a cart carries. A handheld service audits to the guideline like any other, the rate it catches the defects its proof it works, the standard unbent by the size of the probe. The guideline gives a clinic a target to build toward, the same whether it is met on a cart or a probe in a pocket.

About the Author

Julien Mercier

Senior R&D Engineer · Medical Ultrasound Transducer Development

Senior R&D Engineer with an M.S. in Applied Physics and over 15 years of experience in medical ultrasound transducer development, specializing in the design verification and performance testing of high-frequency imaging transducers. Currently leading the development and verification of the company’s next-generation high-frequency linear-array transducer, responsible for imaging performance evaluation and reliability analysis in preclinical testing. Brings extensive hands-on experience in piezoelectric element tuning, beamforming parameter optimization, and system-level performance testing.