By Sarina Gomez
This article is general information, not medical advice. Ask your midwife, OB-GYN or sonographer about your own pregnancy and scans.
To read an ultrasound picture, look at it in three passes: the shades (white is bone, black is fluid, gray is soft tissue), the orientation (the top of the fan is your belly, and you’re seeing a thin slice, not a photo), and the text band (gestational age, due date and measurements). The numbers describe size and dating; what they mean for your pregnancy is your provider’s call, not something to decode at home.
This guide walks through each pass, then decodes the abbreviations most US printouts carry.
First pass: what white, gray and black mean
An ultrasound picture is a map of echoes. The probe sends sound waves into the body and, as ACOG explains, the waves bounce back like echoes and are turned into images. How bright each spot looks depends on how much sound came back from it.
A teaching page from Loyola University Chicago’s medical school sums up the rules:
- White: bone reflects sound strongly. The skull, the spine and the long bones of the arms and legs show up bright, and they can cast a dark “shadow” behind them.
- Black: sound travels through fluid, so fluid looks black. The dark space around your baby is amniotic fluid; the black bubble in the belly is often the stomach.
- Gray: soft tissue, such as the baby’s skin, the placenta and the wall of the uterus, falls in between.
That’s why the face can look skeletal on a 2D print. You’re seeing bright bone edges, not skin and features. A 3D scan builds a shaded surface from many slices, which is why it looks more like a face.
Second pass: why it looks sideways, upside down or mirrored
Most “I can’t tell what I’m looking at” moments come from orientation. Three things help:
- The top of the fan is the probe. On an abdominal scan, the narrow top of the wedge is where the probe touched your skin. Anything near the top is closest to your belly; anything at the bottom is toward your back.
- It’s a slice. The picture is a thin cross-section, like one slice from a loaf. A profile view cuts the baby down the middle from forehead to chin; a round outline of the head is a slice straight across it.
- Left and right depend on the probe. Which side of the screen the head lands on depends on how the sonographer holds the probe and on the machine’s settings. The same baby can face left on one print and right on the next, and neither is a mirror image of how you were lying.
A quick way in: find the brightest curve. It’s usually the skull. From there, look for the forehead, the small bump of the nose and the lips to find a profile, or for a row of bright dots (the spine) running away from the head.
Third pass: the labels and numbers, decoded
The text band at the top or side of the picture holds the clinic details and the measurements. Not every machine shows every label, and clinics vary, but these are the common ones:
| Label | Stands for | What it tells you |
|---|---|---|
| Name, ID, DOB | Patient name, record number, date of birth | Whose scan it is; crop this before sharing online |
| Date and time | When the image was saved | Useful for your keepsake files |
| GA | Gestational age | How far along the pregnancy is, shown in weeks and days (for example 20w3d) |
| LMP | Last menstrual period | The date dating starts from, if you gave one |
| EDD or EDC | Estimated date of delivery (or confinement) | The due date |
| CRL | Crown-rump length | Length from the top of the head to the bottom, used for dating in the first trimester |
| BPD | Biparietal diameter | Width of the head from side to side |
| HC | Head circumference | Distance around the head |
| AC | Abdominal circumference | Distance around the belly |
| FL | Femur length | Length of the thigh bone |
| EFW | Estimated fetal weight | A weight estimate calculated from several measurements |
| FHR or HR | Fetal heart rate | Heartbeats per minute |
| + and dotted lines | Calipers | The two points the sonographer measured between |
| TI and MI | Thermal index and mechanical index | Safety readouts of the machine’s output, not about your baby |
Next to each measurement you may see a week value (such as “BPD 4.71 cm 20w2d”). That’s the age at which that size is typical, not a separate due date.
The TI and MI numbers surprise people. The American Institute of Ultrasound in Medicine says operators monitor these output indices to keep exposure low.
What the numbers are for, and what they aren’t
Measurements are mainly used to date the pregnancy and follow growth. ACOG’s guidance on estimating the due date says a first-trimester ultrasound, up to 13 weeks 6 days, is the most accurate way to establish gestational age, and that CRL dating in that window is accurate to within about 5 to 7 days. In the second trimester, dating relies on formulas that combine BPD, HC, FL and AC.
The same guidance says the due date should be set early and changed only in rare circumstances. So if a later scan shows a measurement a few days “ahead” or “behind”, that doesn’t automatically move your due date.
What a printout can’t tell you:
- Whether a measurement is a concern. That depends on the full exam, your history and the report, which your provider reviews.
- The baby’s sex, reliably. ACOG notes that the sex may be visible if the baby is in a good position, but reading it from a single print at home is guesswork. Early-scan methods like the nub theory have mixed evidence.
- What the face will look like. Fluid, position and the slice angle all change how features appear.
If something on your printout worries you, write the label down and ask at your next visit rather than searching each number.
Questions worth asking your sonographer
The best time to understand your picture is during the scan, while the image is live. Short, specific questions work well:
- “Can you show me where the head, spine and heart are?”
- “Is this view a profile or a cross-section?”
- “Which way is the baby facing right now?”
- “Can you point to the calipers for this measurement?”
- “Can I have a print or digital copy of the profile view?”
In some clinics the sonographer leaves any discussion of results to your doctor or midwife, so save medical questions for them. For what each appointment shows, see ultrasound pictures by week, the 12 week scan and the 20 week anatomy scan.
Turning your picture into a face
Once you can find the profile, you can also imagine the face behind it. SeeMyBaby turns a photo of your printout or a screenshot of your scan into an imagined newborn portrait in about a minute. It doesn’t read your measurements or check anything medical. It works from the picture alone, so a clear 3D or 4D profile gives a closer result than a 2D slice.
It’s for fun, not a prediction. See how the ultrasound to baby portrait works, or read more scan guides on the SeeMyBaby blog, including the difference between a sonogram and an ultrasound.
Sources
- American College of Obstetricians and Gynecologists: Ultrasound exams (FAQ)
- ACOG, AIUM and SMFM, Committee Opinion 700: Methods for estimating the due date
- American Institute of Ultrasound in Medicine: Prudent use and safety of diagnostic ultrasound in pregnancy
- Loyola University Chicago Stritch School of Medicine: Basics of ultrasound
Questions parents ask
How do you read an ultrasound picture for gender?
You can’t do it reliably from a printout. ACOG says the sex may be visible on a standard exam if the baby is in a good position; ask the sonographer during the scan if you want to know.
What do the numbers on an ultrasound picture mean?
They’re the gestational age, due date and measurements such as CRL, BPD, HC, AC and FL, each sometimes paired with the week that size is typical for. Your provider interprets them as part of the full exam.
Why is the baby upside down on my ultrasound?
The top of the image is where the probe touched your belly, so the picture is oriented from the probe’s point of view, not yours. Which side the head appears on also depends on how the probe was held.
What does black mean on an ultrasound picture?
Black usually means fluid, such as the amniotic fluid around the baby, because sound passes through fluid instead of bouncing back.
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