## Screening for Gestational Diabetes Mellitus (GDM) in Second Trimester **Key Point:** The 75 g oral glucose tolerance test (OGTT) is the investigation of choice for GDM screening at 24–28 weeks of gestation in India, as per the National Guidelines. ### Rationale for OGTT at 28 Weeks **High-Yield:** GDM screening is performed at 24–28 weeks because: 1. Insulin resistance peaks in the third trimester. 2. Early detection allows time for dietary and lifestyle interventions. 3. Fetal complications (macrosomia, hypoglycaemia) can be prevented with early management. ### OGTT Protocol for GDM Screening | Step | Details | |------|----------| | **Fasting state** | Not required (universal screening approach) | | **Glucose load** | 75 g anhydrous glucose dissolved in 250–300 mL water | | **Timing of samples** | Fasting (0 min), 1 hour, 2 hours | | **Diagnostic cutoff (WHO 2013)** | Fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥153 mg/dL | | **Interpretation** | Any ONE value ≥ cutoff = GDM positive | **Clinical Pearl:** The 75 g OGTT is a single-step procedure (no need for 50 g screening test first in India) and has replaced the older two-step approach in many guidelines. ### Why OGTT is Superior to Other Tests ```mermaid flowchart TD A[GDM Screening at 24-28 weeks]:::outcome --> B{Investigation choice?}:::decision B -->|OGTT 75g| C[Gold standard<br/>High sensitivity & specificity<br/>WHO recommended]:::action B -->|Fasting + 2-hr glucose| D[Suboptimal<br/>Misses postprandial hyperglycemia<br/>Not standard]:::urgent B -->|Random glucose| E[Insensitive<br/>High false-negative rate<br/>Not recommended]:::urgent B -->|HbA1c| F[Reflects 3-month average<br/>Misses acute GDM<br/>Not suitable for screening]:::urgent C --> G[Positive: Dietary counselling<br/>Negative: Routine follow-up]:::action ``` **Mnemonic:** **OGTT** = **O**ral **G**lucose **T**olerance **T**est (75 g load at 24–28 weeks for GDM detection).
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