What are the routine pregnancy scans?
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routine pregnancy scans schedule first second third trimester 2024
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Here is a full overview of the routine pregnancy scans:
Routine Pregnancy Scans
Pregnancy ultrasound (USS) is non-invasive, carries no known risk to the fetus or mother, and is used for obstetric assessment as well as screening for structural and chromosomal abnormalities. - Emery's Elements of Medical Genetics and Genomics
1. Early Viability Scan (Optional - ~6-10 weeks)
This is not universally offered as a routine scan but may be performed when there is concern about:
- Possible miscarriage or ectopic pregnancy
- Uncertain dates or vaginal bleeding
It confirms the presence of a fetal heartbeat and intrauterine location.
2. First Trimester Dating + Nuchal Translucency (NT) Scan (~11-14 weeks)
This is the first routine scan offered to all pregnant women.
What it assesses:
- Gestational age - measures crown-rump length (CRL) to confirm or establish the expected due date; this dating does not change once established
- Viability - confirms a live pregnancy and rules out missed miscarriage
- Number of fetuses - detects twins or higher-order multiple pregnancies and determines chorionicity (important for managing twin pregnancies)
- Nuchal translucency (NT) - measures the fluid thickness at the back of the fetal neck. Increased NT is associated with Down syndrome (trisomy 21), trisomies 13 and 18, Turner syndrome, triploidy, and a wide range of other fetal abnormalities. In one study, 80% of Down syndrome fetuses had NT above the 95th percentile. - Emery's Elements of Medical Genetics and Genomics, p. 339
Combined first-trimester screening test (10-14 weeks):
NT measurement is combined with two blood tests:
- Free beta-hCG
- PAPP-A (pregnancy-associated plasma protein A)
- Maternal age is factored in
This "combined test" screens for Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). - Emery's Elements of Medical Genetics and Genomics, Table 20.1
3. Second Trimester Anomaly Scan (~18-20 weeks)
This is the most detailed routine scan and the standard for structural assessment.
What it assesses (systematic survey of fetal anatomy):
- Head and brain - ventricles, cerebellum, midline structures, spine
- Face - cleft lip, jaw size (micrognathia), nasal bone
- Heart - four-chamber view, outflow tracts (detects major congenital heart defects)
- Abdomen - stomach, kidneys, bladder, abdominal wall (exomphalos, gastroschisis)
- Limbs - long bone lengths, digits (polydactyly)
- Placenta - position (rules out placenta praevia)
- Amniotic fluid - volume assessment
- Cervical length - sometimes assessed as a predictor of preterm birth
A 20-week fetal anomaly scan is routine and has replaced the older 16-week maternal serum AFP assay as the primary method for detecting structural defects. - Emery's Elements of Medical Genetics and Genomics, p. 321
The NHS Fetal Anomaly Screening Programme (FASP) sets the standard at 18+0 to 20+6 weeks with defined national standards.
Soft markers also assessed at this scan (e.g., choroid plexus cysts, echogenic bowel, short femur) can raise suspicion for chromosomal abnormalities and prompt further testing.
4. Third Trimester Growth Scans (~28-40 weeks)
Not universally offered to low-risk pregnancies in all guidelines, but commonly performed:
| Indication | Timing |
|---|---|
| Routine growth surveillance (in many units) | ~28, 32, and 36 weeks |
| Presentation check (head-down vs breech) | ~36 weeks |
| Suspected growth restriction (IUGR) | Serial scans every 2-4 weeks |
| Placental position re-check (if low at 20 weeks) | ~32-36 weeks |
| Multiple pregnancy surveillance | Every 2-4 weeks from 20 weeks |
Recent evidence suggests 36 weeks is the optimal single screening time in low-risk populations for detecting late adverse outcomes such as growth restriction and abnormal presentation. - PMC review, 2025
Summary Table
| Scan | Gestation | Main Purpose |
|---|---|---|
| Viability scan (if needed) | 6-10 weeks | Confirm heartbeat, location, rule out ectopic |
| Dating + NT scan | 11-14 weeks | Date pregnancy, screen for chromosomal abnormalities |
| Anomaly scan | 18-20 weeks | Detailed structural survey of fetal anatomy |
| Growth scan(s) | 28-36+ weeks | Growth, presentation, placenta, wellbeing |
Additional Scans Based on Risk
Beyond routine scans, additional USS may be offered for:
- Doppler studies - umbilical artery, middle cerebral artery, ductus venosus (in growth restriction)
- Cervical length - in women at risk of preterm labour
- Fetal echocardiography - detailed cardiac scan if anomaly suspected or family history
- Biophysical profile (BPP) - movement, tone, breathing, amniotic fluid assessment in high-risk pregnancies
These complement the routine schedule and are tailored to individual risk factors. The ACOG guidelines on routine prenatal tests provide a good general reference for the standard antenatal schedule.
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