What assessments and screenings should be performed during a 12-week antenatal (prenatal) visit?

12-Week Antenatal Visit Assessment

At the 12-week antenatal visit, perform comprehensive laboratory screening (blood type, antibody screen, complete blood count, hepatitis B, HIV, syphilis, rubella), initiate first-trimester aneuploidy screening with nuchal translucency measurement and biochemical markers (PAPP-A and free β-hCG or total hCG), confirm gestational age by crown-rump length measurement, and assess fetal anatomy to the extent visualized at this gestational age. 1

Laboratory Testing

Essential Blood Work

• Blood type and antibody screen to identify Rh status and potential blood incompatibilities 1
• Complete blood count to detect anemia and establish baseline hematologic parameters 1
• Infectious disease screening: hepatitis B surface antigen, HIV, syphilis (RPR or VDRL), and rubella immunity status 1
• Fasting blood glucose in women with risk factors for pre-existing diabetes (obesity, family history, prior gestational diabetes) 1
• Urinalysis and urine culture to screen for asymptomatic bacteriuria 1

Additional Testing for High-Risk Women

• Thyroid-stimulating hormone in women with pre-existing diabetes or thyroid disease history 1
• Serum creatinine and urinary albumin-to-creatinine ratio in women with chronic hypertension or pre-existing diabetes 1
• Liver enzymes and uric acid in women with chronic hypertension to establish baseline for preeclampsia monitoring 1

Ultrasound Assessment

Dating and Chorionicity

• Crown-rump length (CRL) measurement between 11-13 weeks provides gestational age accuracy within 7 days and is the gold standard for pregnancy dating 2
• Chorionicity determination in twin pregnancies is essential and should be established at this visit, as first-trimester ultrasound has nearly 100% accuracy 3

Nuchal Translucency Screening

• NT measurement should be performed between 11 weeks 0 days and 13 weeks 6 days (or 45-84 mm CRL) as part of first-trimester aneuploidy screening 2
• NT measurement requires certified sonographers and proper technique to ensure accuracy 2
• Combined with maternal age and biochemical markers, NT screening achieves approximately 85% detection rate for Down syndrome at 5% false-positive rate 2

Early Anatomic Survey

• Fetal anatomy visualization at 11-14 weeks can detect approximately 51% of major structural anomalies, with highest detection for neck anomalies (92%) and lowest for limbs, face, and genitourinary defects (34%) 4
• Complete anatomical survey is achievable in approximately 67% of cases at 12-17 weeks, with cardiac anatomy having the lowest completion rate (77%) 5
• Early anatomy scan is particularly valuable in high-risk populations, achieving 83% sensitivity and 95% specificity for fetal anomalies 5
• Kidneys can be visualized in 98% of cases by 11 weeks and 100% by 12 weeks 6

Aneuploidy Screening

First-Trimester Combined Screening

• PAPP-A (pregnancy-associated plasma protein-A) measurement, which increases 40-50% per week between 11-13 weeks 2
• Free β-hCG or total hCG measurement, which decreases 20-40% from 11-13 weeks 2
• Combined with NT and maternal age, this achieves detection rate of approximately 85% for Down syndrome at 5% false-positive rate 2
• Optimal timing is at 11 completed weeks when clinical sensitivity is highest 2

Adjustments to Risk Calculation

• Maternal weight adjustment should be applied, particularly for PAPP-A which shows the strongest correlation with weight 2
• Gestational age should be expressed as weeks and days (e.g., 12 weeks 5 days = 12.7 weeks) for optimal screening performance 2

Nutritional Supplementation

Folic Acid Dosing

• Standard-risk women (BMI < 30 kg/m²): 400 micrograms daily until 12 weeks gestation 2
• High-risk women (BMI ≥ 30 kg/m² or type 2 diabetes): 5 mg folic acid daily until 12 weeks gestation, after checking for vitamin B12 deficiency 2

Aspirin for Preeclampsia Prevention

• Women with BMI ≥ 35 kg/m² may be advised to take 75-180 mg aspirin daily from 12 weeks until delivery to reduce preeclampsia risk, if gastrointestinal bleeding risk is low 2

Post-Bariatric Surgery Patients

• Women with history of bariatric surgery require nutritional screening during each trimester including ferritin, folate, vitamin B12, calcium, vitamin D, and vitamin A 2
• Avoid pregnancy for 12-18 months post-bariatric surgery to allow weight stabilization 2
• Replace vitamin A supplements from retinol to beta-carotene form during pregnancy 2

Clinical Counseling

Lifestyle and Weight Management

• Provide diet and exercise counseling based on BMI and IOM weight gain recommendations 2
• Recommend 150 minutes per week or 30 minutes daily of moderate-intensity exercise 2
• Discuss importance of controlling weight gain during pregnancy, particularly in women with obesity 2

Screening Results Discussion

• Explain the meaning of first-trimester screening results, including that screen-negative results do not guarantee a normal fetus 2
• Discuss options for diagnostic testing (CVS at 10-13 weeks) if screening is positive 2
• Provide genetic counseling referral when indicated (e.g., family history of chromosomal abnormalities) 2

Special Populations

Twin Pregnancies

• Chorionicity determination is mandatory and should be established at this visit 3
• Counsel about increased risks: 5-fold increase in fetal death, 7-fold increase in neonatal death compared to singletons 3
• Plan for nuchal translucency scan at 11-14 weeks and detailed anatomic survey at 18-22 weeks 3
• Discuss need for more intensive monitoring including regular third-trimester growth scans 3

Women with Chronic Hypertension

• Establish comprehensive baseline including complete blood count, liver enzymes, renal function, and uric acid to detect superimposed preeclampsia later in pregnancy 1

Common Pitfalls to Avoid

• Failing to establish accurate dating by CRL at this visit, which is critical for all subsequent screening and management decisions 2
• Missing the window for NT measurement, which must be performed between 11+0 and 13+6 weeks 2
• Inadequate folic acid dosing in high-risk women (obesity, diabetes), who require 5 mg rather than 400 micrograms 2
• Not determining chorionicity in twin pregnancies at this optimal timing when accuracy is nearly 100% 3
• Delaying glucose screening in women with risk factors for pre-existing diabetes, which should be performed at the first prenatal visit 1