A 28-year-old woman from Mumbai attends the antenatal clinic reporting irregular menstrual cycles. She is unsure of her last menstrual period and states it was 'sometime in the last 3 months.' Transabdominal ultrasound is performed for dating. The fetal biometry shows: head circumference 24 cm, femur length 4.2 cm, biparietal diameter 8.5 cm, and abdominal circumference 18 cm. There is no fetal cardiac activity. What is the most likely gestational age and diagnosis?

Explanation

## Diagnosis: Intrauterine Fetal Demise at ~16 Weeks Gestation ### Gestational Age Assessment by Biometry | Parameter | Value (Case) | Expected at 16 weeks | Expected at 14 weeks | Expected at 12 weeks | |-----------|--------------|----------------------|----------------------|----------------------| | **HC (cm)** | 24 | 23–25 | 20–22 | 17–19 | | **FL (cm)** | 4.2 | 4.0–4.5 | 3.5–3.8 | 2.8–3.2 | | **BPD (cm)** | 8.5 | 8.0–8.8 | 7.2–7.8 | 6.4–7.0 | | **AC (cm)** | 18 | 17–19 | 15–17 | 13–15 | **High-Yield:** All four parameters cluster at **16 weeks gestation**, with excellent agreement (±1–2 days accuracy at this gestational age). ### Diagnosis: Intrauterine Fetal Demise (IUFD) **Key Point:** Absence of fetal cardiac activity in a fetus with biometry consistent with 16 weeks gestation (well beyond the 11–14 week window where cardiac activity is expected) confirms **intrauterine fetal demise**. ### Differential Diagnosis at Different Gestational Ages ```mermaid flowchart TD A["No Fetal Cardiac Activity"]:::outcome --> B{"Gestational Age?"}:::decision B -->|"< 11 weeks"| C["Anembryonic pregnancy<br/>or very early loss"]:::outcome B -->|"11–14 weeks"| D["Missed miscarriage<br/>or early IUFD"]:::outcome B -->|"> 14 weeks"| E["Intrauterine fetal demise<br/>Requires delivery planning"]:::urgent C --> F["Expectant or medical management"]:::action D --> G["Expectant, medical, or surgical management"]:::action E --> H["Induction of labor<br/>or surgical evacuation"]:::action ``` ### Why This Case = IUFD at 16 Weeks 1. **Biometry consensus:** All four parameters (HC, FL, BPD, AC) converge on **16 weeks ± 3–5 days**. 2. **Cardiac activity absent:** At 16 weeks, fetal heart should be clearly visible on transabdominal ultrasound (visible from ~11 weeks with transvaginal probe, ~13–14 weeks with transabdominal). 3. **Definition:** IUFD is loss of fetal cardiac activity after 20 weeks in most guidelines, but loss of cardiac activity in a viable-age fetus (> 14 weeks) with normal biometry is termed **fetal demise**. 4. **Clinical significance:** This requires delivery planning (induction of labor or surgical evacuation), not expectant management alone. **Clinical Pearl:** In the second trimester, **HC and FL are the most accurate dating parameters** (±3–5 days). If all parameters agree at 16 weeks, the gestational age is 16 weeks regardless of the patient's reported LMP. **Mnemonic: DATING ACCURACY BY TRIMESTER** **1st trimester:** ±3–5 days (CRL most accurate) **2nd trimester:** ±1–2 weeks (HC, FL, BPD) **3rd trimester:** ±3–4 weeks (all parameters less reliable) [cite:Callen's Ultrasonography in Obstetrics and Gynecology 6e Ch 3]