Ultrasound in Pregnancy: When, What It Shows and Results
Reviewed by the LabReadAI medical team
Ultrasound is the main way to see how the baby grows and develops. It is done several times during pregnancy, and the report leaves the mother with cryptic CRL, BPD, AC, AFI. Let's break down when ultrasound is done in pregnancy, what it shows and how to read the main values.
Ultrasound Timing in Pregnancy (Screenings)
There are usually three routine scans, one per trimester:
- 11–14 weeks (1st screening): confirming dates, assessing the risk of chromosomal anomalies (with biochemistry — see pregnancy screening), nuchal translucency.
- 18–21 weeks (2nd screening): detailed fetal anatomy, placenta, amniotic fluid.
- 30–34 weeks (3rd screening): fetal growth and position, placenta, blood flow.
Outside the schedule, ultrasound is done for complaints (pain, bleeding) or on indication.
What a Fetal Ultrasound Shows
Ultrasound assesses: the heartbeat and its rate, fetal size (fetometry), organ anatomy, placental position and maturity, amniotic fluid volume, blood flow (Doppler). Early on, it confirms an intrauterine pregnancy and the dates; this matters when an ectopic pregnancy is suspected.
Reading the Main Values
| Abbreviation | What it is |
|---|---|
| CRL | crown-rump length (dating in the 1st trimester) |
| NT | nuchal translucency (anomaly risk) |
| BPD | biparietal diameter of the head |
| HC / AC | head / abdominal circumference |
| FL | femur length |
| AFI | amniotic fluid index |
| FHR | fetal heart rate |
All sizes are compared with norms for the dates — small deviations are often a normal variant; the trend and the whole picture matter.
Is Ultrasound Safe and Is Preparation Needed
Ultrasound in pregnancy is safe (no radiation). Early on, a transvaginal scan may need an empty bladder; later, transabdominal scans need no special preparation. The baby's sex is usually visible from the 2nd screening — more in baby's sex on ultrasound.
When a Result Needs Attention
Tell your doctor about fetometry deviating from the dates, impaired blood flow, low/high amniotic fluid, placenta praevia or a low-lying placenta, or absent heartbeat. Ultrasound complements but does not replace monitoring and tests — the overall plan is in what tests are done in pregnancy.
To understand your ultrasound report in plain language, upload it (PDF or photo) to the imaging interpretation service: the AI will explain the values for the dates. This helps you understand the result but does not replace an obstetrician.
This article is informational. Pregnancy care and ultrasound interpretation are the doctor's job.
For informational purposes only
This article is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Please consult a healthcare professional for medical guidance.