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Summary Table

Category	Details
Risk Factors	Obesity (high pre-pregnancy BMI), age >25 years, family history of DM in first-degree relatives, history of macrosomia, history of glucose intolerance, glucosuria, Asian/Hispanic/African American/Pacific Islander ethnicity, high gestational weight gain, smoking
Etiology	Pre-existing beta-cell dysfunction unmasked by the physiologic insulin resistance of pregnancy; impaired suppression of hepatic glucose production; peripheral insulin resistance worsened by placental hormones (hPL, progesterone, cortisol, prolactin)
Patient Presentation	Usually asymptomatic; detected on routine screening at 24-28 weeks. Occasionally presents with polyuria, polydipsia, or recurrent candidal infections. May present late with polyhydramnios or LGA fetus on ultrasound
Classic Physical Exam	No pathognomonic exam finding. Fundal height greater than expected for gestational age (suggesting macrosomia/polyhydramnios). Acanthosis nigricans may suggest underlying insulin resistance
Key Diagnostic Results	75 g OGTT (PERKENI standard): FPG ≥92 mg/dL, 1-hr ≥180 mg/dL, or 2-hr ≥153 mg/dL (venous, fasting). At ≥24 weeks, 1 abnormal value is sufficient. At <24 weeks, 2 abnormal values are required
Management	Medical nutrition therapy (MNT) + structured physical activity first. If glycemic targets not met in 2-4 weeks, start insulin (first-line pharmacotherapy). Metformin is a second-line option in the third trimester only
Keywords	"Glucose intolerance first recognized in pregnancy," "large for gestational age," "polyhydramnios," "neonatal hypoglycemia," "macrosomia," "24-28 weeks screening"

1. Pathophysiology

Gestational diabetes mellitus arises from an inability of the maternal pancreatic beta cells to compensate for the physiologic insulin resistance that develops during pregnancy. This insulin resistance is driven by placental hormones, primarily human placental lactogen (hPL), progesterone, cortisol, and prolactin, all of which increase progressively through the second and third trimesters. In a normal pregnancy, the beta cells undergo compensatory hyperplasia and increase insulin secretion by 2- to 3-fold to maintain euglycemia.

In women who develop GDM, there is a pre-existing defect in beta-cell function, specifically a blunted first-phase insulin secretion, that predates the pregnancy. This subclinical dysfunction is clinically silent until the escalating insulin resistance of late pregnancy overwhelms the limited secretory reserve. The result is maternal hyperglycemia, particularly in the postprandial state.

The liver also contributes. In GDM, the effectiveness of insulin in suppressing hepatic glucose production is reduced compared to normal pregnancy, leading to elevated fasting glucose levels. Peripheral insulin resistance at the level of skeletal muscle is also greater, especially in women who were obese before conception. Catalano et al. demonstrated that pre-conception peripheral insulin resistance independently worsens insulin sensitivity during a GDM pregnancy.

The consequence for the fetus follows the Pedersen hypothesis: maternal hyperglycemia crosses the placenta freely via facilitated diffusion (GLUT transporters), but maternal insulin does not. The fetal pancreas responds to this glucose load by producing excess insulin (fetal hyperinsulinemia). Insulin is an anabolic hormone and a growth factor in utero, so the hyperinsulinemic fetus develops macrosomia (or LGA), increased adiposity, organomegaly, and is at risk for neonatal hypoglycemia once the placental glucose supply is abruptly cut at delivery.

The risk of developing GDM is modulated by obesity, advanced maternal age, and genetic background. Ethnic groups with high baseline DM prevalence, including Asian populations, are classified as at least moderate risk, which is why PERKENI 2021 recommends universal screening for all pregnant women in Indonesia.

2. Classification & Clinical Manifestation

The PERKENI 2021 guideline distinguishes two categories of hyperglycemia in pregnancy:

Category	Definition	Onset of Hyperglycemia	Severity	Insulin Requirement
Diabetes Mellitus Gestational (DMG / GDM)	Glucose intolerance first recognized during pregnancy	Typically detected at 24-28 weeks (second trimester onward)	Generally milder hyperglycemia	Not always; lifestyle modification may suffice
Diabetes Mellitus with Pregnancy (DMH / Pre-gestational DM)	Pre-existing DM type 1 or type 2 with subsequent pregnancy	Present before conception or in the first trimester	Generally more severe; may have macrovascular/microvascular complications	Insulin required from diagnosis onward

This distinction matters because DMH carries the additional burden of pre-existing end-organ damage (retinopathy, nephropathy, cardiovascular disease) that complicates both the pregnancy and the delivery.

Short-Term and Long-Term Complications of GDM

Timeframe	Maternal Complications	Fetal/Neonatal Complications
Short-term	Preeclampsia/eclampsia, difficult labor, cesarean delivery, preterm delivery	Neonatal hypoglycemia, neonatal hyperbilirubinemia, LGA/macrosomia, shoulder dystocia, perinatal intensive care
Long-term	GDM in subsequent pregnancies, progression to DM type 2, coronary heart disease	Obesity, DM type 2, hypertension, coronary heart disease

The critical teaching point here is that GDM is not just a pregnancy problem. It is a window into future metabolic risk for both the mother and the child. The intrauterine hyperglycemic environment is thought to epigenetically reprogram the fetal metabolic profile, predisposing the offspring to obesity and type 2 DM later in life.

3. Diagnostic Workup

Step	Test	Purpose	Timing
Screening (all pregnant women)	Risk factor assessment (Table 2 criteria) + random blood glucose (GDS), venous or capillary	Stratify risk and decide who needs immediate OGTT	First antenatal visit, regardless of gestational age
Best Initial Diagnostic Test	75 g Oral Glucose Tolerance Test (OGTT), venous blood, fasting (standard procedure)	Confirm or rule out GDM	24-28 weeks (or earlier if high risk)
Alternative 1	75 g OGTT using capillary blood, fasting	For facilities without venous lab capability	Same timing
Alternative 2	75 g OGTT without fasting, venous or capillary	For patients unable to fast (remote areas, practical barriers)	Same timing
If GDS ≥200 mg/dL at screening	Proceed to DM type 2 diagnostic workup	Rule out overt DM type 2 in pregnancy	Immediately at any gestational age

Diagnostic Values

Method	Fasting Plasma Glucose	1-hour Post-75g	2-hour Post-75g
Standard OGTT (venous, fasting)	≥92 mg/dL	≥180 mg/dL	≥153 mg/dL
Alternative 1 (capillary, fasting)	≥95 mg/dL	≥191 mg/dL	≥162 mg/dL
Alternative 2 (venous, non-fasting)	Not applicable	≥180 mg/dL	≥153 mg/dL
Alternative 2 (capillary, non-fasting)	Not applicable	≥191 mg/dL	≥162 mg/dL

Diagnostic Criteria by Gestational Age

Gestational Age	Number of Abnormal Values Required
≥24 weeks	At least 1 abnormal value on OGTT
<24 weeks	At least 2 abnormal values on OGTT

Step 1: Screening. At the first antenatal visit, every pregnant woman undergoes risk stratification using clinical characteristics (obesity, family history of DM, prior macrosomia, ethnicity, history of glucose intolerance, glucosuria) and a random blood glucose (GDS) check. This produces three pathways:

Moderate risk + GDS <140 mg/dL: Educate on healthy lifestyle. Recheck GDS monthly. If GDS rises to ≥140 mg/dL at any point, proceed to OGTT.
Moderate risk + GDS 140-199 mg/dL, OR High risk + GDS ≤199 mg/dL: Proceed to OGTT for GDM diagnosis within the same week.
GDS ≥200 mg/dL regardless of risk category: This is not GDM. Proceed to a DM type 2 diagnostic workup. If confirmed, manage as DMH (pre-gestational diabetes with pregnancy).

Step 2: OGTT procedure (standard). Three days before the test, the patient eats her usual diet with adequate carbohydrates and maintains normal activity. She fasts 8-10 hours overnight, then a fasting venous glucose is drawn the next morning.

Before proceeding with the glucose load, the fasting glucose result is interpreted based on gestational age:

If FPG <92 mg/dL at any gestational age: continue OGTT with 75 g glucose load.
If FPG 92-125 mg/dL and gestational age ≥24 weeks: OGTT does not need to continue; GDM is diagnosed.
If FPG 92-125 mg/dL and gestational age <24 weeks: continue OGTT (need a second abnormal value).
If FPG >126 mg/dL at any gestational age: stop OGTT; proceed to DM type 2 workup.

After the fasting sample, 75 g of glucose dissolved in 300 mL of water (preferably warm to reduce nausea) is consumed within 5 minutes. Blood is drawn again at 1 hour and 2 hours post-load.

Why PERKENI 2021 offers alternatives. The standard OGTT requires venous blood draw capability and an 8-10 hour fast, which are logistically difficult in remote regions of Indonesia where health facilities are limited and patients travel long distances. The capillary alternative (Alternative 1) and the non-fasting alternative (Alternative 2) use higher diagnostic cutoffs derived from the HAPO study at the OR 2.0 threshold (rather than OR 1.75 for standard values), which compensates for the reduced precision of capillary glucometry and the added variability of non-fasting states.

If GDM is not diagnosed at <24 weeks, repeat the OGTT at 24-28 weeks.

4. Management & Treatment

Component	Details
First-line	Medical nutrition therapy (MNT) + structured physical activity + self-monitoring of blood glucose (SMBG) + weight gain monitoring
Glycemic targets	FPG and pre-meal <95 mg/dL; 1-hr postprandial <140 mg/dL; 2-hr postprandial <120 mg/dL
Pharmacotherapy (if lifestyle fails in 2-4 weeks)	Insulin is first-line. Basal: NPH or detemir. Prandial: regular insulin, aspart, or lispro
Second-line pharmacotherapy	Metformin, considered only in the third trimester if MNT + activity fail after 2 weeks. Start 500 mg 1-2x/day, titrate every 10 days, max 2000 mg/day. Crosses the placenta. ~40% eventually need added insulin
Intrapartum	Maintain maternal glucose 70-140 mg/dL during labor to minimize neonatal hypoglycemia
Postpartum	Breastfeed immediately and for ≥6 months. Repeat OGTT or HbA1c at 6 weeks to 6 months postpartum. If negative, recheck annually for 3 years, then every 3 years thereafter

Medical Nutrition Therapy (MNT)

The minimum daily macronutrient intake for a pregnant woman with GDM is 175 g carbohydrates, 71 g protein, 56 g fat, and 28 g fiber. Carbohydrates should not exceed 50% of total caloric intake, and preference is given to complex carbohydrates with a low glycemic index to blunt postprandial glucose spikes and limit pathologic insulin resistance.

Total daily caloric requirement is calculated using pre-pregnancy weight (PERKENI adopts the Japanese recommendation):

Trimester 1: 25-30 kcal/kg + 50 kcal
Trimester 2: 25-30 kcal/kg + 250 kcal
Trimester 3: 25-30 kcal/kg + 450 kcal

If postprandial hyperglycemia persists despite adequate total calories, the standard three-meal schedule is redistributed into 4-6 smaller meals per day. Adding a small nighttime snack also reduces the risk of starvation ketosis, which can occur more readily in pregnancy due to accelerated maternal fat catabolism during fasting.

Supplementation includes vitamin C, vitamin E, vitamin D (improves insulin sensitivity and calcium absorption), selenium, iron, and omega-3 fatty acids.

Structured Physical Activity

Exercise is initiated at 12 weeks and continues until 38-39 weeks. The recommended regimen combines moderate-intensity aerobic exercise (walking, stationary cycling, swimming, low-impact aerobic dance) with light-to-moderate resistance training, totaling 150 minutes per week.

Type	Duration/Frequency	Intensity	Avoid
Aerobic (low-impact)	20-30 min/day, 3-5 days/week	Moderate: target heart rate per age group; patient can talk clearly but cannot sing	High-intensity exercise, long-distance running, contact sports, scuba diving, activities with fall risk
Resistance training	15-20 min/day, 3-5 days/week	Light to moderate: 1-3 kg weights, 10-15 repetitions, up to 2 sets	Isometric exercises, Valsalva maneuver, heavy weights, hot yoga/pilates, supine floor exercises
Pelvic floor (Kegel)	10-15 min/day, 3-5 days/week	Target 100 repetitions per session	N/A

Contraindications to exercise include hemodynamically significant cardiovascular disease, restrictive lung disease, cervical incompetence, multiple gestation at risk for premature labor, placenta previa after 26 weeks, ruptured membranes, preeclampsia, and impaired intrauterine growth.

Self-Monitoring of Blood Glucose (SMBG)

For patients managed with MNT, activity, and/or metformin: check fasting glucose and 1-hour or 2-hour postprandial glucose (after the largest meal), each twice per week.

For patients on basal-bolus insulin: more frequent monitoring is required (before and after every meal).

Weight Gain Monitoring

PERKENI 2021 adapts the IOM 2009 guidelines using WHO Asia-Pacific BMI cutoffs:

Pre-pregnancy BMI	Weekly Weight Gain (Trimester 2-3)	Total Gestational Weight Gain
Underweight (<18.5 kg/m²)	0.51 kg (0.44-0.58)	12.5-18 kg
Normal (18.5-22.9 kg/m²)	0.42 kg (0.35-0.50)	11.5-16 kg
Overweight (23.0-24.9 kg/m²)	0.28 kg (0.23-0.33)	7-11.5 kg
Obese (>25 kg/m²)	0.22 kg (0.17-0.27)	5-9 kg

Note the lower BMI thresholds compared to the standard IOM cutoffs (which use 25 for overweight and 30 for obese). This adjustment reflects the higher metabolic risk profile of Asian populations at lower BMI levels.

Pharmacotherapy

Insulin is the first-line pharmacologic agent. It is initiated if glycemic targets are not met after 2-4 weeks of optimized MNT and physical activity.

Basal insulin (NPH or detemir): targets fasting and pre-meal glucose.
Prandial insulin (regular, aspart, or lispro): targets postprandial glucose.
Insulin can also be started immediately if there is evidence of a large fetus or polyhydramnios with FPG ≥108 mg/dL.
Dose titration follows the standard DM insulin consensus.

Metformin is a second-line option with conditions: it may be considered only when MNT + activity have been adequately attempted for 2 weeks without reaching targets and the pregnancy has entered the third trimester. The patient and partner must be informed that metformin crosses the placenta. Starting dose is 500 mg once or twice daily, titrated every 10 days, up to a maximum of 2000 mg/day. Approximately 40% of patients on metformin will eventually require supplemental insulin.

No other oral antidiabetic agents are recommended during pregnancy due to insufficient safety data.

Postpartum Care

Insulin is discontinued after delivery in GDM (it typically resolves).
Breastfeeding should begin immediately and continue for at least 6 months (reduces neonatal hypoglycemia risk, lowers maternal risk of future DM type 2, and reduces childhood obesity risk).
A 75 g OGTT or HbA1c is performed at 6 weeks to 6 months postpartum using standard (non-pregnancy) diagnostic criteria.
If negative: repeat annually for 3 consecutive years, then every 3 years.
Long-acting reversible contraception (IUD or progestin implant) is preferred during the monitoring period.
Preconception counseling (nutrition, activity, normal BMI maintenance) should begin early if future pregnancy is planned.

5. Differential Diagnosis & Distractors

Differential	Why It Looks Similar	Key Discriminator
Pre-gestational DM type 2 (DMH)	Also presents with hyperglycemia in pregnancy; may be discovered incidentally during first antenatal visit	FPG >126 mg/dL or GDS ≥200 mg/dL at any gestational age, especially in the first trimester. GDM by definition is glucose intolerance first arising in pregnancy, typically detected at 24-28 weeks. PERKENI routes GDS ≥200 to a DM type 2 workup, not GDM
Pre-gestational DM type 1	Hyperglycemia in pregnancy requiring insulin	Usually known before pregnancy; history of autoimmune disease, low C-peptide, positive anti-GAD/IA-2 antibodies, often younger patient, insulin-dependent from onset
Transient hyperglycemia of stress	Elevated random glucose during acute illness or steroid use in pregnancy	Resolves when the stressor is removed; does not meet OGTT criteria; no postprandial pattern of hyperglycemia
Impaired glucose tolerance (non-pregnant criteria)	Glucose values above normal but may not meet GDM thresholds	Pregnancy-based diagnostic cutoffs (IADPSG/PERKENI) are lower than standard IGT criteria. A 2-hr glucose of 140-152 mg/dL is "normal" by non-pregnant IGT standards but is abnormal by standard GDM criteria at 153 mg/dL. Always apply pregnancy cutoffs during pregnancy
Preeclampsia	Shares risk factors with GDM (obesity, advanced maternal age); both are complications of pregnancy	Preeclampsia is defined by hypertension (≥140/90) + proteinuria or end-organ dysfunction after 20 weeks. Glucose tolerance is normal. However, GDM is a risk factor for preeclampsia, so the two can coexist

6. Traps & High-Yield Pearls

The most common way students lose points on GDM questions is by confusing GDM with pre-existing (pre-gestational) DM discovered during pregnancy. When a vignette presents a woman at her first antenatal visit with a random glucose ≥200 mg/dL or a fasting glucose >126 mg/dL, the correct move is to pursue a DM type 2 diagnostic pathway, not to label it GDM. The PERKENI guideline explicitly separates this group and directs them to be managed as DMH, which carries a different management protocol (insulin from the start, preconception complication screening, stricter monitoring).

A second trap involves the number of abnormal OGTT values needed for diagnosis depending on gestational age. At ≥24 weeks, only one abnormal value is required. At <24 weeks, two abnormal values are required. A vignette may present a woman at 20 weeks with a single borderline-elevated 2-hour glucose and ask whether GDM is confirmed. The answer is no; a second abnormal parameter must be present, or the OGTT should be repeated at 24-28 weeks.

Third, students frequently choose metformin as the initial pharmacologic agent for GDM because it is the go-to drug for type 2 DM. In pregnancy, insulin is the first-line pharmacologic therapy. Metformin is a conditional second-line option reserved for the third trimester, and even then it crosses the placenta and ~40% of patients end up needing insulin anyway.

Fourth, the glycemic targets during labor (70-140 mg/dL) are tested as a clinical scenario: a laboring GDM patient with a glucose of 160 mg/dL and the question asks for the next best step. The answer is insulin adjustment to bring glucose into range, because intrapartum hyperglycemia directly causes neonatal hypoglycemia via the Pedersen mechanism (fetal hyperinsulinemia persists after cord clamping while the glucose supply stops).

Finally, postpartum follow-up is a commonly tested long-term management point. GDM resolves after delivery, but these women carry a lifetime risk of developing DM type 2. The OGTT must be repeated at 6 weeks to 6 months postpartum using non-pregnancy diagnostic criteria, and surveillance continues annually for 3 years, then every 3 years. Forgetting this follow-up schedule is a frequent exam error.

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