A 32-year-old multigravida at 20 weeks of gestation presents for her routine antenatal visit. She reports no complaints. Obstetric examination is unremarkable. Her first-trimester combined screening (NT + biochemistry) showed a low risk for chromosomal abnormalities (risk 1 in 2500). She has no significant past medical history and is not diabetic. What is the most appropriate next investigation at this visit?

Question

Accepted Answer

C. Detailed morphology ultrasound scan. ## Second-Trimester Antenatal Investigations at 20 Weeks

### Gestational Age and Screening Context
At 20 weeks of gestation, the patient is in the mid-second trimester. She has already completed first-trimester combined screening with a reassuring result (low risk). The next standard investigation is the detailed morphology scan.

### Detailed Morphology Ultrasound Scan

**Key Point:** The detailed morphology scan (anatomy scan) is performed at 18–22 weeks of gestation to systematically assess fetal anatomy, detect structural abnormalities, and confirm fetal viability and growth.

**High-Yield:** The morphology scan evaluates:
- Fetal biometry (head circumference, biparietal diameter, femur length, abdominal circumference)
- Cardiac anatomy (four-chamber view, outflow tracts)
- Cerebral anatomy (lateral ventricles, cerebellum, nuchal fold)
- Spinal anatomy
- Abdominal organs (stomach, kidneys, bladder)
- Limbs and digits
- Placental location and cord insertion
- Amniotic fluid volume
- Maternal uterine artery Doppler (in some centres)

### Why Morphology Scan at 20 Weeks?

**Clinical Pearl:** The 20-week morphology scan is a cornerstone of antenatal care. It is the single best opportunity to detect major structural fetal abnormalities (e.g., neural tube defects, cardiac defects, renal abnormalities) that may influence counselling, management, and delivery planning. It is standard of care in all pregnancies, regardless of screening risk.

### Investigation Timing Algorithm

```mermaid
flowchart TD
  A[Pregnancy confirmed]:::outcome --> B[11-14 weeks: First-trimester combined screening]:::action
  B --> C{Risk assessment}:::decision
  C -->|Low risk| D[18-22 weeks: Morphology scan]:::action
  C -->|High risk| E[Consider NIPT or invasive testing]:::action
  D --> F[24-28 weeks: Glucose tolerance test]:::action
  F --> G[28-34 weeks: Doppler if indicated]:::action
```

### Comparison of Investigations at Different Gestations

| Gestation | Investigation | Purpose | Indication |
|---|---|---|---|
| 11–14 weeks | First-trimester combined screening (NT + biochemistry) | Chromosomal risk assessment | Universal screening |
| 15–22 weeks | Quad marker screening | Second-trimester biochemical screening | If first-trimester screening not done |
| 18–22 weeks | **Detailed morphology scan** | **Structural abnormality detection** | **Universal; standard of care** |
| 24–28 weeks | OGTT (75 g) | Gestational diabetes screening | Universal screening |
| 28–34 weeks | Doppler studies | Placental insufficiency assessment | High-risk pregnancies (PIH, IUGR, previous adverse outcome) |

**Mnemonic:** **SCAN** = **S**tructure assessment **C**onfirms **A**natomy at **N**ineteen weeks (18–22).

### Why Not the Other Options?

**Oral Glucose Tolerance Test (OGTT):** Gestational diabetes screening is typically performed at 24–28 weeks of gestation, not at 20 weeks. Screening at 20 weeks is premature and not recommended.

**Repeat Quad Marker Screening:** The patient has already completed first-trimester combined screening with a low-risk result. Repeating biochemical screening is unnecessary and does not add clinical value.

**Doppler Studies:** Doppler assessment of umbilical and middle cerebral arteries is reserved for high-risk pregnancies (e.g., pre-eclampsia, IUGR, previous adverse outcome) and is typically performed in the third trimester or when clinically indicated. This patient has no risk factors.

Answer

A. Doppler studies of umbilical and middle cerebral arteries

Answer

B. Repeat quad marker screening

Answer

D. Oral glucose tolerance test (OGTT) with 75 g glucose

Explanation

Second-Trimester Antenatal Investigations at 20 Weeks

Gestational Age and Screening Context

Detailed Morphology Ultrasound Scan

Why Morphology Scan at 20 Weeks?

Investigation Timing Algorithm

Comparison of Investigations at Different Gestations

Why Not the Other Options?

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Gestation	Investigation	Purpose	Indication
11–14 weeks	First-trimester combined screening (NT + biochemistry)	Chromosomal risk assessment	Universal screening
15–22 weeks	Quad marker screening	Second-trimester biochemical screening	If first-trimester screening not done
18–22 weeks	Detailed morphology scan	Structural abnormality detection	Universal; standard of care
24–28 weeks	OGTT (75 g)	Gestational diabetes screening	Universal screening
28–34 weeks	Doppler studies	Placental insufficiency assessment	High-risk pregnancies (PIH, IUGR, previous adverse outcome)