What are the differential diagnoses to consider during a routine prenatal visit at 24 weeks gestation?

Differential Diagnoses at 24 Weeks Gestation Routine Prenatal Visit

At a routine 24-week prenatal visit, the primary differential diagnoses to actively screen for include gestational diabetes mellitus, preeclampsia risk assessment, fetal growth abnormalities, multiple gestation complications, and fetal anatomic anomalies.

Metabolic and Endocrine Conditions

Gestational Diabetes Mellitus

• All pregnant women not previously diagnosed with diabetes should undergo screening at 24-28 weeks gestation using either a one-step 75-g OGTT or two-step approach with 50-g glucose challenge test 1, 2.
• Women with BMI ≥30 kg/m² who had negative early screening require repeat testing at this visit 3.
• The 24-28 week window represents the optimal time for detecting gestational diabetes, as insulin resistance peaks during this period 1.

Pre-existing Diabetes (Previously Undiagnosed)

• Women presenting for their first prenatal visit at 24 weeks or later should be tested for overt diabetes using fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) or A1C ≥6.5% 1.
• If overt diabetes is not found at initial assessment at 24 weeks or later, a 75-g OGTT should follow 1.

Hypertensive Disorders

Preeclampsia

• At every assessment after 20 weeks, screen for new hypertension, new proteinuria, headache, visual disturbances, epigastric pain, and vomiting 1.
• Women with one predisposing factor (nulliparity, age ≥40, BMI ≥35, family history, multiple pregnancy, chronic hypertension, renal disease) require assessment no more than every 3 weeks between 24-32 weeks 1.
• Risk factors carry substantial relative risks: antiphospholipid antibodies (RR 9.72), previous preeclampsia (RR 7.19), pre-existing diabetes (RR 3.56), multiple pregnancy (RR 2.93) 1.

Chronic Hypertension with Superimposed Preeclampsia

• Women with pre-existing hypertension require vigilant monitoring for new proteinuria or worsening symptoms indicating superimposed preeclampsia 4.

Fetal Anatomic Abnormalities

Major Congenital Malformations

• The 20-week anatomic ultrasound (typically performed 18-22 weeks) should have been completed, but if delayed until 24 weeks, major structural anomalies must be evaluated 1, 5.
• Approximately 3% of fetuses have major congenital malformations detectable by ultrasound 5.

Isolated Soft Markers for Aneuploidy

• If isolated soft markers are detected on ultrasound at this visit (echogenic intracardiac focus, echogenic bowel, urinary tract dilation, shortened long bones, thickened nuchal fold), counseling should address aneuploidy risk 1.
• With negative prior screening (serum or cell-free DNA), isolated echogenic intracardiac focus or choroid plexus cysts require no further evaluation as they are normal variants 1.
• Isolated echogenic bowel warrants evaluation for cystic fibrosis and fetal cytomegalovirus infection, plus third-trimester growth assessment 1.

Multiple Gestation Complications

Twin-Twin Transfusion Syndrome

• Multiple pregnancies identified earlier require ongoing surveillance for complications specific to chorionicity 1.
• Multiple pregnancy carries 2.93 times increased risk for preeclampsia 1.

Discordant Growth in Twins

• Serial growth assessments are necessary to detect growth discordance between twins.

Fetal Growth Abnormalities

Intrauterine Growth Restriction

• Assessment for reduced fetal movements or size less than dates should trigger evaluation for growth restriction 1.
• Doppler ultrasound of umbilical artery and middle cerebral artery can assess blood flow in suspected growth restriction 5.

Macrosomia

• Undiagnosed or poorly controlled diabetes increases risk of excessive fetal growth.

Placental Abnormalities

Placenta Previa

• If placental location was low-lying on earlier ultrasound, reassessment at 24 weeks determines persistence of previa.

Placental Insufficiency

• May manifest as growth restriction or abnormal Doppler studies.

Cervical Insufficiency

Short Cervix/Preterm Birth Risk

• Women with risk factors for preterm delivery may benefit from transvaginal cervical length assessment around this gestational age 6.
• History of prior preterm birth or second-trimester loss increases suspicion.

Infectious Complications

Cytomegalovirus Infection

• Consider if isolated echogenic bowel is detected on ultrasound 1.

Other Congenital Infections

• Syphilis, toxoplasmosis, and other infections may manifest with fetal findings at this gestation.

Hematologic Conditions

Anemia

• Physiologic anemia of pregnancy peaks in second trimester; symptomatic anemia requires evaluation 2.

Alloimmunization

• Women with positive antibody screens require ongoing monitoring for fetal anemia.

Critical Screening Pitfalls to Avoid

• Do not skip gestational diabetes screening at 24-28 weeks, as this is the evidence-based window for detection 1, 2.
• Do not dismiss isolated soft markers without proper risk assessment and counseling, particularly thickened nuchal fold or absent nasal bone which carry higher aneuploidy risk 1.
• Do not overlook preeclampsia screening in women with risk factors, as monitoring frequency should be intensified (every 2-3 weeks) 1.
• Do not delay anatomic survey beyond 24 weeks, as detection of major anomalies allows for informed decision-making before viability thresholds 1, 7.