Gestational diabetes mellitus (GDM) is diabetes first diagnosed in the second or third trimester that was not clearly present before pregnancy. Pregestational diabetes (type 1 or type 2 present before pregnancy, or diagnosed early in pregnancy) carries higher risks, especially for birth defects.

Why GDM develops

• In the first half of pregnancy, insulin sensitivity is normal or increased; nausea plus insulin use can cause hypoglycemia
• In the second half, placental hormones — human placental lactogen (hPL), progesterone, estrogen, cortisol, placental growth hormone — cause insulin resistance so more glucose reaches the fetus
• If the pancreas cannot increase insulin output enough, GDM results. It usually resolves after the placenta is delivered, but it signals a high lifetime risk of type 2 diabetes

Classification: A1GDM — controlled with nutrition and exercise; A2GDM — requires medication.

Risk factors: overweight or obesity, physical inactivity, previous GDM, previous macrosomic infant (≥ 4,000 g / 8.8 lb), first-degree relative with diabetes, high-risk race or ethnicity (including Asian, Hispanic, Black, Native American, Pacific Islander), PCOS, hypertension, prediabetes, older maternal age.

Effects of maternal hyperglycemia

• Usually no symptoms — found by screening
• Glycosuria alone is not diagnostic (normal renal threshold falls in pregnancy)
• Fundal height larger than dates, excessive weight gain, polyhydramnios on ultrasound
• Recurrent vaginal yeast infection or UTI
• Self-monitoring records, diet, activity, and insulin technique at each visit
• Hypoglycemia signs (sweating, shakiness, tachycardia, confusion) in clients using insulin

Early pregnancy (first visit) — test clients with risk factors for undiagnosed pregestational diabetes using standard criteria (e.g., A1C ≥ 6.5%, fasting glucose ≥ 126 mg/dL (7.0 mmol/L)). ACOG does not recommend routine early testing for GDM itself.

24–28 weeks — all pregnant clients not already diagnosed

Two-step approach (preferred by ACOG in the US)

1. 50-g glucose challenge test, nonfasting; 1-hour plasma glucose ≥ 130, 135, or 140 mg/dL (7.2, 7.5, or 7.8 mmol/L) — the cutoff depends on the institution → proceed to step 2
2. 100-g 3-hour OGTT, fasting — GDM if 2 or more values meet or exceed the thresholds

One-step approach (IADPSG/WHO criteria; widely used outside the US; ACOG allows by clinical judgment) — 75-g 2-hour OGTT, fasting; GDM if any one value is met or exceeded: fasting 92 mg/dL (5.1 mmol/L), 1 hour 180 mg/dL (10.0 mmol/L), 2 hours 153 mg/dL (8.5 mmol/L).

OGTT preparation: overnight fast of 8–14 hours, usual carbohydrate intake for several days beforehand, stay seated, no smoking or eating during the test.

Monitoring during pregnancy

• Self-monitoring of blood glucose: fasting and 1 or 2 hours after each meal (4 times daily)
• Targets (ACOG/ADA): fasting < 95 mg/dL (5.3 mmol/L), 1-hour post-meal < 140 mg/dL (7.8 mmol/L), or 2-hour post-meal < 120 mg/dL (6.7 mmol/L)
• For type 1 or type 2 diabetes treated with insulin, the ADA also gives lower limits: fasting 70–95 mg/dL (3.9–5.3 mmol/L), 1 hour 110–140 mg/dL (6.1–7.8 mmol/L), 2 hours 100–120 mg/dL (5.6–6.7 mmol/L); with CGM in type 1 diabetes, aim for more than 70% of time at 63–140 mg/dL (3.5–7.8 mmol/L)
• A1C target: < 6% if achievable without significant hypoglycemia (< 6.5% preconception and in the first trimester for pregestational diabetes); A1C is a secondary measure in pregnancy
• Ultrasound for fetal growth and amniotic fluid; antenatal fetal testing (nonstress tests) for A2GDM and pregestational diabetes, usually from about 32 weeks

Medical nutrition therapy (first-line for all)

• Individualized plan with a dietitian; carbohydrates spread over 3 meals and 2–3 snacks (including a bedtime snack to prevent overnight ketosis)
• Minimum about 175 g carbohydrate, 71 g protein, and 28 g fiber daily; choose complex, high-fiber carbohydrates; limit sweets, juices, and refined starches; carbohydrate is not eliminated
• Weight gain within IOM ranges for BMI

Physical activity — about 30 minutes of moderate activity most days; walking after meals lowers post-meal glucose

Medication (A2GDM) — when targets are not met

Hypoglycemia (same thresholds as nonpregnant adults): Level 1 < 70 mg/dL (3.9 mmol/L); Level 2 < 54 mg/dL (3.0 mmol/L); Level 3 = severe event needing assistance. If conscious and able to swallow: 15 g fast-acting carbohydrate, recheck in 15 minutes, repeat if still < 70 mg/dL (3.9 mmol/L), then a snack. If unable to swallow: glucagon or IV dextrose. Pregnant clients with type 1 diabetes have a higher risk of severe hypoglycemia, especially in the first trimester; family members should know how to use glucagon.

Pregestational diabetes — preconception care (A1C < 6.5% if safe, folic acid, review medications — stop ACE inhibitors, ARBs, statins, and GLP-1 receptor agonists before conception as directed); insulin is the preferred drug; low-dose aspirin from 12–16 weeks (preeclampsia prevention); early dilated eye exam; baseline kidney function and urine albumin.

Timing of birth (ACOG)

• A1GDM, well controlled: 39 0/7–40 6/7 weeks
• A2GDM, well controlled: 39 0/7–39 6/7 weeks
• Poorly controlled or with complications: earlier, individualized
• Suspected very large fetus (≥ 4,500 g) in diabetes: cesarean may be offered

Labor and birth — hourly capillary glucose; intrapartum target about 70–110 mg/dL (3.9–6.1 mmol/L) with IV insulin and dextrose infusions as needed (insulin-treated clients).

Postpartum

• Insulin needs fall abruptly after the placenta is delivered. GDM: usually stop medication and check glucose. Pregestational: lower doses (often below prepregnancy doses at first) — watch closely for hypoglycemia, especially with breastfeeding
• Encourage breastfeeding — helps maternal glucose and lowers the child's future risk
• Screen for type 2 diabetes with a 75-g 2-hour OGTT at 4–12 weeks postpartum (ACOG 2024 also allows testing during the delivery hospitalization), then every 1–3 years for life

Other common medical conditions

Listed in priority order.

1. Detect and treat hypoglycemia in insulin-treated clients (Rule of 15; glucagon or IV dextrose if unable to swallow)
2. Recognize DKA in pregestational type 1 diabetes (nausea, vomiting, abdominal pain, Kussmaul breathing) — urgent, because DKA can occur at glucose near 200 mg/dL (11.1 mmol/L) in pregnancy and threatens the fetus
3. Monitor glucose and fetal well-being — review glucose logs, fundal height, fetal movement, antenatal testing
4. Teach self-management — meter use, nutrition, insulin technique, activity
5. Newborn care: early feeding within the first hour, neonatal glucose checks per protocol, skin-to-skin contact, monitor for respiratory distress and jaundice
6. Arrange postpartum diabetes testing and communicate the lifelong risk

• GDM is common and manageable; good control lowers risks for the baby
• Check glucose as instructed and record results; bring the log to every visit
• Eat regular meals and snacks; do not skip meals or cut out carbohydrates; limit sugary drinks and juice
• Walk after meals if cleared by the provider
• Insulin: rotate sites, do not massage the site, store unopened insulin in the refrigerator (never freeze), carry fast-acting glucose
• Know hypoglycemia symptoms and the Rule of 15
• Report persistent high readings, vomiting, decreased fetal movement
• After birth: breastfeed, keep the postpartum glucose test appointment, maintain a healthy weight and activity, and get tested before any future pregnancy — GDM is likely to recur and type 2 diabetes risk remains high

• GDM develops in the second half of pregnancy because placental hormones (hPL) cause insulin resistance
• Screen all at 24–28 weeks; screen early only for undiagnosed pregestational diabetes in high-risk clients
• Two-step: 50-g nonfasting → 100-g 3-hour fasting OGTT (2 abnormal values); one-step: 75-g OGTT (1 abnormal value)
• Targets: fasting < 95 mg/dL (5.3 mmol/L); 1 h < 140 mg/dL (7.8 mmol/L); 2 h < 120 mg/dL (6.7 mmol/L)
• Diet: 3 meals + 2–3 snacks, including bedtime; carbohydrate not eliminated
• Insulin is preferred — does not cross the placenta; metformin and glyburide do
• Insulin needs rise in late pregnancy and drop sharply after the placenta is delivered
• Macrosomia → shoulder dystocia; newborn → hypoglycemia (feed early, check glucose), RDS, jaundice
• Birth defects are linked to pregestational diabetes, not GDM
• Postpartum 75-g OGTT at 4–12 weeks, then every 1–3 years
• Hypothyroidism: raise levothyroxine dose when pregnancy is confirmed; hyperthyroidism: PTU in first trimester