How to Read Pregnancy Ultrasound Abbreviations

Pregnancy ultrasound reports are packed with abbreviations like CRL, BPD, HC, AC, FL, AFI, and EFW, and most of them are never explained to you before the report lands in your hands. Each abbreviation represents a specific measurement or assessment that tells your provider something about your baby’s size, age, anatomy, or well-being. Research on social media posts by pregnant women found that acronyms on ultrasound reports were one of the most common sources of confusion and anxiety, with many women turning to online forums for help decoding them.¹ This guide walks through the abbreviations you’re most likely to see, grouped by what they actually measure and when they matter most.

Early Pregnancy Measurements

The first ultrasound abbreviations you’ll encounter are the ones used to figure out how far along you are and whether the pregnancy is developing on track. The most common early terms include:

GS: Gestational sac. This is the fluid-filled structure that surrounds the embryo in very early pregnancy. It’s often the first thing visible on ultrasound, sometimes before the embryo itself can be seen.
mGSD: Mean gestational sac diameter. This is the average of three measurements of the sac, used to estimate gestational age before the embryo is large enough to measure directly.
CRL: Crown-rump length. Once the embryo is visible, this measurement from the top of the head to the bottom of the torso becomes the most accurate way to date the pregnancy. It’s used from about six to fourteen weeks.
GA: Gestational age, typically expressed in weeks and days.
EDD or EDC: Estimated due date (or estimated date of confinement, an older term that means the same thing).
LMP: Last menstrual period. Your provider uses this date as a starting point for dating, but the ultrasound measurements may adjust it.

The relationship between the gestational sac and the embryo matters clinically. If the embryo (measured by CRL) is almost as large as the sac itself, leaving very little space between them, it can signal a higher risk of early pregnancy loss. Research in IVF patients found that when the difference between the mean sac diameter and the CRL was less than 5 mm, the first-trimester loss rate was roughly 44%, compared with about 7% when that gap was 15 mm or more.² You probably won’t see “mGSD minus CRL” spelled out on your report, but if your provider mentions that the sac is small relative to the embryo, this is what they’re assessing.

Dating discrepancies between LMP-based and ultrasound-based gestational age also carry clinical weight. A large prospective study found that when the ultrasound-based age lagged behind the LMP-based age by more than three days, the risk of pregnancy loss increased roughly fivefold, and a lag of more than five days was associated with a greater than sixfold increase.³ This doesn’t mean that every discrepancy signals a problem; irregular cycles, late ovulation, and normal biological variation all contribute. But it explains why your provider might redate your pregnancy based on the CRL rather than your LMP.

Second and Third Trimester Growth Measurements

Once you’re past the first trimester, the CRL becomes less useful because the baby starts curling and flexing. From roughly 14 weeks onward, your reports will shift to a standard set of four measurements used to track fetal growth and estimate gestational age:

BPD: Biparietal diameter. This is the distance across the widest part of the baby’s skull, measured from one parietal bone to the other. Think of it as a side-to-side head measurement.
HC: Head circumference. The measurement around the baby’s head at the same level as the BPD, giving a fuller picture of head size.
AC: Abdominal circumference. A measurement around the baby’s belly at the level of the liver and stomach. This one is especially sensitive to growth problems because it reflects how much fat and liver glycogen the baby is storing.
FL: Femur length. The length of the baby’s thighbone, the longest bone in the body.

These four measurements are used both individually and together. Individually, each one is compared to reference charts that show expected ranges for a given gestational age.⁴ Combined, they’re plugged into mathematical formulas to generate the estimated fetal weight, which usually appears as EFW on your report. The most widely used formula, developed by Hadlock, uses head circumference, abdominal circumference, and femur length to calculate the weight.⁵

Keep in mind that EFW is an estimate with a margin of error. It can be off by 10 to 15% in either direction, and that margin gets wider at the extremes of fetal size. If your report says the baby weighs 2,500 grams, the actual weight could reasonably be anywhere from about 2,100 to 2,900 grams. Providers know this and use EFW as one piece of a larger picture, not as a scale reading.

What a Short Femur Length Means

If your report flags a short femur length (sometimes written as “FL below the 5th percentile”), it can be unsettling. In isolation, a short FL is often just a normal variant, especially in parents who are shorter themselves. However, research has found that isolated short femur in the second half of pregnancy is associated with higher rates of small-for-gestational-age babies and growth restriction.⁶ Your provider may recommend additional monitoring to track growth over time, and population and ethnic differences in bone length are considered when interpreting these numbers.

Percentiles on Your Report

You’ll often see each measurement reported alongside a percentile. A BPD at the 50th percentile means the baby’s head width falls right in the middle of what’s expected for that gestational age. Values between roughly the 10th and 90th percentiles are generally considered normal. A measurement below the 10th percentile doesn’t automatically mean something is wrong, but it prompts your provider to look at the trend across multiple ultrasounds. A single measurement is a snapshot; the trajectory over time tells the real story.

First Trimester Screening Abbreviations

If you have a first-trimester screening ultrasound (typically done between 11 and 14 weeks), your report may include some terms related to chromosomal risk assessment:

NT: Nuchal translucency. This is the measurement of a fluid-filled space at the back of the baby’s neck. A thicker-than-expected NT can be associated with chromosomal conditions and certain heart defects.
NB: Nasal bone. The presence or absence of a visible nasal bone at the first-trimester scan is noted because an absent nasal bone is more common in pregnancies affected by trisomy 21 (Down syndrome).

Both of these markers increase the sensitivity of first-trimester screening for chromosomal abnormalities when combined with blood tests.⁷ Some advanced screening protocols also assess the baby’s heart valves and a blood vessel called the ductus venosus (abbreviated DV), adding terms like TR (tricuspid regurgitation) to the report.⁸ These are specialized markers you’d typically see at centers offering detailed early screening. The blood test abbreviations that often accompany them include PAPP-A (pregnancy-associated plasma protein A) and fβ-hCG (free beta human chorionic gonadotropin), which are serum markers, not ultrasound measurements, though they may appear on the same report.

Second Trimester Soft Markers

The detailed anatomy scan, usually done around 18 to 22 weeks, may flag what are called “soft markers.” These are minor ultrasound findings that are usually harmless on their own but can be associated with chromosomal abnormalities when multiple are present. The abbreviations you might encounter include:

EIF: Echogenic intracardiac focus, a small bright spot in the baby’s heart. This is one of the most commonly noted soft markers.
CPC: Choroid plexus cyst, a small fluid-filled area in the brain that almost always resolves on its own.
SUA: Single umbilical artery. Normally the umbilical cord contains two arteries and one vein (sometimes written as “3-vessel cord”). A two-vessel cord with only one artery is flagged because it can be associated with structural differences in the baby’s urinary, cardiovascular, or digestive systems.⁹

A single soft marker in an otherwise normal anatomy scan is very often clinically insignificant. The concern increases when multiple soft markers appear together. A study examining over 600 fetuses with soft markers found that echogenic intracardiac focus and the presence of multiple markers were the findings most strongly correlated with chromosomal abnormalities, with the overall rate of chromosomal issues in that group being about 11%.¹⁰ If your report mentions a single EIF or a single CPC and nothing else looks unusual, it’s rarely a cause for alarm. Your provider will put the finding in the context of your overall risk profile, including your age and any blood test results.

Amniotic Fluid Abbreviations

Your provider keeps an eye on how much amniotic fluid surrounds the baby throughout pregnancy. Too little (oligohydramnios) or too much (polyhydramnios) can signal complications. Two main methods are used to measure it, and both show up as abbreviations on reports:

AFI: Amniotic fluid index. The ultrasound tech divides your abdomen into four quadrants and measures the deepest pocket of fluid in each one, then adds the four numbers together. A normal AFI is typically between 5 and 24 cm.
SDP or DVP: Single deepest pocket (also called deepest vertical pocket). Instead of measuring four quadrants, the tech finds the single deepest pocket of fluid and measures it. A normal SDP is generally between 2 and 8 cm.

The distinction between AFI and SDP matters more than you might think. A Cochrane review found that using the AFI method diagnosed significantly more cases of oligohydramnios compared with using the single deepest pocket method, and this led to more labor inductions and cesarean deliveries for fetal distress, without improving actual outcomes for the baby.¹¹ In other words, the AFI approach can overdiagnose low fluid and lead to interventions that may not have been necessary. A multicenter trial defined oligohydramnios as an AFI of 5 cm or below, or the absence of a pocket measuring at least 2 × 1 cm when using the SDP method.¹² If your report shows a low AFI, it’s worth asking your provider whether the SDP measurement was also considered.

Cervical Length

Cervical length (CL) may appear on your report if you have a transvaginal ultrasound to check the cervix, especially if you have risk factors for preterm birth. A normal cervix in the second trimester is generally 30 mm or longer. A short cervix, typically defined as 25 mm or less, is associated with an increased risk of preterm delivery, and the risk climbs steeply as the measurement drops.¹³ Women with a cervical length of 10 mm or less face the highest risk and tend to have the shortest interval to delivery.

In women experiencing symptoms of preterm labor, cervical length measurement can help predict who will actually deliver within a week, which helps providers decide who needs aggressive intervention and who can safely be observed.¹⁴ If your CL measurement is on the shorter side, you may see terms like “funneling” (the internal opening of the cervix beginning to dilate) or “dynamic cervix” (changes in length with pressure). Treatments for a short cervix can include progesterone supplementation or a cervical cerclage.

Placenta Location and Related Terms

Your ultrasound report will describe where the placenta is located. Common descriptions include anterior (front wall of the uterus), posterior (back wall), fundal (top of the uterus), or lateral (side wall). The location itself usually doesn’t matter much for the pregnancy, with one major exception: placenta previa, where the placenta covers or nearly covers the cervix.

If placenta previa or a low-lying placenta is found on transabdominal ultrasound, guidelines recommend a transvaginal ultrasound to precisely define the placental edge location, including which side of the cervix it’s on and whether there are additional findings like a succenturiate lobe (an accessory lobe of placenta) or cord insertion near the cervix.¹⁵ A low-lying placenta found in the second trimester often resolves on its own as the uterus grows, so a follow-up scan is standard.

You may also see placental grading on the report, described as Grade 0 through Grade III. This classification describes the ultrasound appearance of the placenta as it matures. A Grade 0 placenta looks smooth and homogeneous, typical of early pregnancy, while a Grade III placenta shows calcifications and other changes typical of late pregnancy. Early placental maturation (a Grade III placenta before 36 weeks, for instance) can sometimes be associated with growth restriction, but the grading system is used less frequently today than it once was.

Doppler Abbreviations

Doppler ultrasound measures blood flow through specific vessels, and the abbreviations it generates can be particularly confusing. The most common ones include:

UA: Umbilical artery. Doppler of this vessel assesses blood flow from the baby to the placenta.
MCA: Middle cerebral artery. This vessel in the baby’s brain is measured to check whether the baby is redirecting blood flow to protect the brain (a phenomenon sometimes called “brain sparing”).
UtA: Uterine artery. These are maternal vessels assessed to gauge how well blood is flowing to the placenta from the mother’s side.
PI: Pulsatility index. A calculated number reflecting how much resistance to blood flow exists in the vessel being measured.
RI: Resistance index. Similar to the PI, but calculated slightly differently.
S/D ratio: Systolic-to-diastolic ratio, another way to express resistance to flow.

In a healthy pregnancy, resistance in the uterine arteries drops as the placenta develops. When this doesn’t happen, the PI and RI values remain high, and this pattern can signal risk for preeclampsia or fetal growth restriction.¹⁶ A meta-analysis of uterine artery Doppler for predicting preeclampsia reported that the pulsatility index had a sensitivity of about 65% and a specificity of about 88%.¹⁷ You may also see the term “notching,” which refers to a dip in the Doppler waveform of the uterine artery. Notching in the second trimester is considered another marker of impaired blood supply to the placenta.

The ratio of MCA to UA Doppler values (MCA/UA ratio, sometimes called the cerebroplacental ratio or CPR) is used in high-risk pregnancies to assess how the baby is coping with compromised blood flow. Research in pregnancies complicated by preeclampsia and high blood pressure found that babies of mothers with an abnormal MCA/UA ratio had significantly lower birth weights, higher rates of NICU admission, and substantially greater risk of poor outcomes compared with those who had normal ratios.¹⁸ If you see “absent end-diastolic flow” or “reversed end-diastolic flow” (AEDF or REDF) on a Doppler report of the umbilical artery, these are more serious findings that typically trigger close surveillance or delivery discussions.

Biophysical Profile

The biophysical profile (BPP) is a combined assessment that uses ultrasound plus fetal heart-rate monitoring to evaluate the baby’s well-being, typically in the third trimester or in high-risk pregnancies. The BPP evaluates five components: fetal movement, fetal tone (does the baby flex and extend limbs?), fetal breathing movements, amniotic fluid volume, and a non-stress test (NST) measuring the baby’s heart rate over a 20-minute window.¹⁹ Each component is scored 0 or 2, for a maximum score of 10. A score of 8 or 10 is reassuring. A score of 6 is considered equivocal and usually prompts retesting or closer monitoring, and a score of 4 or below raises concern.

A “modified BPP” (mBPP) is a shortcut version that combines just the NST with the amniotic fluid measurement (usually the SDP). It’s faster and used for routine surveillance when a full BPP isn’t deemed necessary. If you see BPP or mBPP on your report, the accompanying score is the number to pay attention to.

Twin Pregnancy Terminology

Twin pregnancies come with their own layer of abbreviations, mostly centered on how the babies share (or don’t share) membranes and placenta:

DCDA: Dichorionic diamniotic. Two separate placentas and two separate amniotic sacs. This is the most common type of twin pregnancy and carries the lowest risk profile.
MCDA: Monochorionic diamniotic. One shared placenta but two separate sacs. These twins share a blood supply and require more monitoring.
MCMA: Monochorionic monoamniotic. One shared placenta and one shared sac. The rarest and highest-risk type due to the possibility of cord entanglement.

Monochorionic twins make up about 20% of twin pregnancies but account for roughly 30% of pregnancy-related complications.²⁰ Chorionicity and amnionicity are ideally determined in the first trimester when ultrasound accuracy is highest. You may see the “lambda sign” or “twin peak sign” noted on your report as evidence of dichorionic placentation, and a “T sign” as evidence of monochorionic placentation. Conditions specific to monochorionic twins that may appear on reports include TTTS (twin-twin transfusion syndrome), TAPS (twin anemia-polycythemia sequence), and TRAP (twin reversed arterial perfusion sequence).²¹

Why the Terminology Can Feel So Opaque

Part of the problem is that ultrasound reporting evolved to serve communication between specialists, not between providers and patients. The Society of Radiologists in Ultrasound has worked to standardize first-trimester terminology, convening a multisociety panel that used a consensus process to develop preferred terms, define synonyms, and identify terms that should be avoided.²² That lexicon distinguishes, for example, between an intrauterine pregnancy (IUP, a pregnancy in its normal location) and an ectopic pregnancy (EP, one implanted abnormally). But even these standardization efforts are aimed at getting radiologists and obstetricians on the same page, not at making reports immediately legible to the patient reading them at home.

Research analyzing social media discussions by pregnant women found that medical terminologies and acronyms were responsible for a substantial share of unmet information needs, and over a quarter of posts expressing negative emotions about ultrasound were tied to confusion, anxiety, or worry about what the report meant.¹ If you’re staring at your report feeling lost, you’re in good company. The most productive next step is usually to bring the specific abbreviation or number that’s worrying you to your next appointment and ask your provider to walk you through it in context, because an abbreviation in isolation often sounds more alarming than it is once the full picture is explained.