• Often performed at 11 to 13 weeks + 6 days as part of first-trimester assessment.
• It may be done earlier, around 6 to 10 weeks, if there is pain, bleeding, uncertain menstrual dates, or a need to confirm viability/location.

1. Viability: fetal cardiac activity.
2. Location: confirms an intrauterine pregnancy and helps exclude ectopic pregnancy.
3. Number of fetuses: singleton or multiple pregnancy, with chorionicity assessment in twins.
4. Gestational age and EDD: primarily by crown-rump length (CRL).
5. Early fetal anatomy: a limited review for major obvious abnormalities.
6. At 11-13+6 weeks, it can be combined with nuchal translucency measurement for aneuploidy screening.

• CRL is measured from the top of the fetal head (crown) to the buttocks (rump).
• Up to 13 weeks + 6 days, CRL is the most accurate ultrasound parameter for dating, generally within about 5-7 days.
• If the scan estimate differs substantially from dates calculated using the last menstrual period, the ultrasound-based EDD is generally preferred because early embryonic growth is relatively uniform.

• Transabdominal ultrasound is commonly sufficient later in the first trimester.
• Transvaginal ultrasound provides better detail in very early pregnancy or when the abdominal scan is unclear.

Congenital rubella syndrome (CRS)

Risk by gestation

• Highest during the first 12 weeks of pregnancy.
• Risk falls progressively thereafter.
• Major structural anomalies are uncommon after 20 weeks, although sensorineural hearing loss can still occur.

Classical triad

1. Eye defects
   • Cataract, often bilateral
   • Congenital glaucoma
   • Pigmentary retinopathy, microphthalmia
2. Cardiac defects
   • Patent ductus arteriosus (PDA) is classic
   • Peripheral pulmonary artery stenosis
   • May have ventricular septal defect
3. Sensorineural deafness
   • The commonest and sometimes the only late manifestation.

Other manifestations

• Intrauterine growth restriction and low birth weight
• Microcephaly, developmental delay, meningoencephalitis
• Hepatosplenomegaly and hepatitis
• Thrombocytopenic purpura with petechiae, producing “blueberry muffin” lesions due to extramedullary hematopoiesis
• Jaundice, anemia
• Interstitial pneumonitis
• Radiolucent bone lesions

Diagnosis

• Rubella-specific IgM in serum supports congenital infection because maternal IgM does not cross the placenta.
• Rubella virus detection by RT-PCR or viral culture from nasopharyngeal/throat swab, urine, blood, or CSF may confirm infection.
• Evaluate hearing, eyes, heart, growth, blood count, liver function, and neurodevelopment.

Prevention

• Prevention is by rubella-containing vaccine, usually MMR, given before pregnancy.
• MMR is a live attenuated vaccine and is not administered during pregnancy.
• A nonimmune pregnant woman should receive MMR postpartum.
• Maternal exposure or a rubella-like illness in pregnancy needs urgent obstetric and infectious-disease assessment, with rubella IgG, IgM, and where appropriate IgG avidity testing.

Two-step approach

1. 50-g oral glucose challenge test (GCT)
   • No fasting required.
   • Measure venous plasma glucose 1 hour after 50 g oral glucose.
   • A value of ≥140 mg/dL (7.8 mmol/L) is commonly considered screen-positive. Some centers use 130 or 135 mg/dL to increase sensitivity.
2. If screen-positive: 100-g, 3-hour oral glucose tolerance test (OGTT)
   • This is the confirmatory diagnostic test, done fasting.
   • GDM is usually diagnosed when 2 or more values are abnormal using the selected criteria.

One-step alternative

• 75-g, 2-hour OGTT, performed fasting at 24-28 weeks.
• This is used in many guidelines/settings as both screening and diagnosis.