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# Second- and third-trimester ultrasound: what it checks

- Reading

Later pregnancy scans check the baby's anatomy, growth, the amount of fluid around it and where the placenta lies.

Updated Nov 13, 2026

## Key points

- Anatomy scan at about 18 to 22 weeks; dates come from the first trimester.
- Abdominal circumference is the most sensitive measurement for growth restriction.
- Low-lying placenta: within 2 cm of the os; most resolve, recheck at about 32 weeks.
- No digital vaginal exam in antepartum bleeding until previa is excluded.
- Absent or reversed end-diastolic flow in the umbilical artery is an urgent finding.

## What it is

After the first trimester the obstetric scan is done mainly through the abdomen, with a transvaginal view added for the cervix and a low placenta. The detailed anatomy scan, usually at about 18 to 22 weeks, looks at each fetal organ system, the placenta, the cord and the fluid. Later scans are done for a reason: to follow growth when the uterus measures small or large or the mother has a risk factor, to recheck a low placenta, to confirm presentation, or to assess fetal well-being with fluid and Doppler. Biometry (head, abdomen and femur) gives an estimated fetal weight that is plotted on a growth chart; the dates are not changed by late scans if an early scan exists.

## What to check

- Number, cardiac activity and presentation (cephalic, breech, transverse) come first: they change management on their own.
- Biometry: biparietal diameter and head circumference, abdominal circumference, femur length; combined into an estimated fetal weight.
- Abdominal circumference is the measurement most sensitive to growth restriction and to macrosomia, because it reflects the fetal liver.
- Late-pregnancy biometry has an error of roughly 15 percent for weight and several weeks for dating, so trends between scans matter more than one value.
- Amniotic fluid: deepest vertical pocket (normal about 2 to 8 cm) or amniotic fluid index (four quadrants, normal about 5 to 24 cm).
- Placenta: low-lying when the edge is within 2 cm of the internal os; previa when it covers the os.
- Cervix: transvaginal cervical length; a short cervix in the mid-trimester predicts preterm birth.

## Reading it step by step (and why)

- Confirm number of fetuses, heartbeat and presentation — they decide what happens next before any measurement.
- Measure head, abdomen and femur and plot the estimated weight on a growth chart — one value is weak; the percentile and its trend count.
- In the anatomy scan, go system by system: head, face, spine, heart, abdomen, kidneys, bladder, limbs — a fixed order finds anomalies.
- Locate the placenta and its lower edge; if near the os, add a transvaginal view — transvaginal scanning is safe in previa and more accurate.
- Look for signs of accreta in a low anterior placenta after caesarean — lost clear zone, lacunae, a thin or broken bladder wall.
- Measure the amniotic fluid — too little points to growth restriction, ruptured membranes or kidney problems; too much to diabetes or anomalies.
- If growth is small, add umbilical artery Doppler — rising resistance shows a failing placenta; absent or reversed diastolic flow is grave.
- Measure cervical length transvaginally when preterm birth is a concern — a short cervix guides prevention and referral.
- Compare with earlier scans and the dates from the first trimester — growth is judged on true gestational age.

## Common mistakes

- Redating a pregnancy from a late scan hides growth restriction; keep the early dates.
- A normal scan does not exclude placental abruption: ultrasound misses most abruptions, which remain a clinical diagnosis.
- Most low-lying placentas in the mid-trimester move away from the os as the lower segment grows; recheck at about 32 weeks rather than label previa early.
- Never do a digital vaginal examination in a woman with bleeding until a scan has excluded placenta previa.
- Vasa previa (fetal vessels over the os) is found only if colour Doppler is applied over the cervix in at-risk pregnancies.
- Fetal growth restriction is usually defined by estimated weight or abdominal circumference below the 10th percentile; small but healthy fetuses also fall there, so Doppler and fluid help separate them.

## Next step

- Estimated weight or abdominal circumference below the 10th percentile: umbilical artery Doppler, fluid assessment and serial growth scans; refer to the obstetric team.
- Absent or reversed end-diastolic flow in the umbilical artery: same-day senior obstetric review.
- Placenta covering the os or low-lying in the mid-trimester: repeat transvaginal scan at about 32 weeks; no digital exam.
- Low anterior placenta with prior caesarean: ask for a targeted scan for accreta and plan delivery at a centre with blood bank and surgical support.
- Suspected anomaly on the anatomy scan: refer to maternal–fetal medicine for detailed scan and counselling.

## Test yourself

Think of the answer, then tap the question to see it.

## Which single biometric measurement is most sensitive for detecting fetal growth restriction?

The abdominal circumference, because it reflects the size of the fetal liver, which shrinks first when the placenta fails.

## When is a placenta called low-lying rather than previa?

When its edge is within 2 cm of the internal os without covering it; previa covers the os.

## Why should a third-trimester scan not be used to change the due date?

Late biometry varies by weeks between fetuses, so redating would hide growth restriction or overgrowth; early dates are kept.

## Does a normal ultrasound exclude placental abruption?

No. Ultrasound misses most abruptions; the diagnosis is clinical (pain, bleeding, a tense uterus, fetal distress).

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/radiology/second-and-third-trimester-ultrasound-what-it-checks/)

## Elsewhere in DurgiPedia

- [Leopold maneuvers](https://durgical.com/en/clinical-exam/leopold-maneuvers/)
- [Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesis](https://durgical.com/en/embryology/prenatal-diagnosis-ultrasound-serum-screening-cell-free-dna-cvs-amniocentesis/)

## Connections

## Related 4

- [First-trimester ultrasound: intrauterine pregnancy, dating and ectopic](https://durgical.com/en/radiology/first-trimester-ultrasound-intrauterine-pregnancy-dating-and-ectopic/)
- [Imaging in pregnancy: what is safe](https://durgical.com/en/radiology/imaging-in-pregnancy-what-is-safe/)
- [Ultrasound: echogenicity, probes, Doppler and artefacts](https://durgical.com/en/radiology/ultrasound-echogenicity-probes-doppler-and-artefacts/)
- [Pelvic ultrasound in gynaecology: cysts, fibroids, torsion](https://durgical.com/en/radiology/pelvic-ultrasound-in-gynaecology-cysts-fibroids-torsion/)

## Related conditions 8

- [Fetal growth restriction](https://durgical.com/en/conditions/fetal-growth-restriction/)
- [Placenta praevia](https://durgical.com/en/conditions/placenta-praevia/)
- [Placenta accreta spectrum](https://durgical.com/en/conditions/placenta-accreta-spectrum/)
- [Vasa previa](https://durgical.com/en/conditions/vasa-previa/)
- [Placental abruption](https://durgical.com/en/conditions/placental-abruption/)
- [Preterm labour and birth](https://durgical.com/en/conditions/preterm-labour-and-birth/)
- [Cervical insufficiency](https://durgical.com/en/conditions/cervical-insufficiency/)
- [Multiple pregnancy and twin–twin transfusion syndrome](https://durgical.com/en/conditions/multiple-pregnancy-and-twin-twin-transfusion-syndrome/)

[← All of radiology](https://durgical.com/en/radiology/)

Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: [Antepartum Care in the Second and Third Trimester](https://www.ncbi.nlm.nih.gov/books/NBK448166/) (StatPearls (NCBI Bookshelf), 2024); [Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation](https://www.ncbi.nlm.nih.gov/books/NBK572062/) (StatPearls (NCBI Bookshelf), 2023); [Fetal Growth Restriction](https://www.ncbi.nlm.nih.gov/books/NBK562268/) (StatPearls (NCBI Bookshelf), 2024); [Placenta Previa](https://www.ncbi.nlm.nih.gov/books/NBK539818/) (StatPearls (NCBI Bookshelf), 2023).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/radiology/second-and-third-trimester-ultrasound-what-it-checks/