What are the implications of a nuchal translucency (NT) of 1mm and a yolk sac size of 0.5 cm on an ultrasound (US)?

NT 1mm and Yolk Sac 5mm: Normal First Trimester Findings

Both measurements are reassuring and within normal limits for early pregnancy, indicating a viable intrauterine pregnancy with low risk for chromosomal abnormalities or adverse outcomes.

Nuchal Translucency Assessment

Your NT measurement of 1mm is well below the threshold for concern and represents a normal finding:

• An increased NT is defined as ≥3mm or above the 99th percentile for crown-rump length 1
• NT of 1mm places this fetus at low risk for chromosomal abnormalities, as approximately one-third of fetuses with NT ≥3mm have chromosomal abnormalities 1, 2
• This normal NT measurement provides reassurance but should not be used in isolation - NT assessment alone detects only 70% of trisomy 21 fetuses 1, 3

Clinical Implications of Normal NT

• First trimester combined screening (NT plus maternal serum markers) detects approximately 90% of affected pregnancies at a 5% false-positive rate 1
• A normal NT significantly reduces but does not eliminate the risk of aneuploidy, particularly when combined with maternal serum screening 1

Yolk Sac Evaluation

Your yolk sac measurement of 5mm (0.5cm) is within the normal range:

• The yolk sac is the first sonographic feature confirming an intrauterine pregnancy and appears as a thin-walled, spherical structure with an anechoic center 1
• The largest yolk sac in viable pregnancies was 8.1mm in a study of normal singleton pregnancies 4
• Your measurement of 5mm falls comfortably within normal limits and suggests a viable pregnancy

Yolk Sac as a Prognostic Indicator

• Abnormal yolk sac findings associated with pregnancy loss include: absent yolk sac, irregular shape, relatively large size (>8mm), or progressive regression 4
• When embryonic heartbeats exist, poor quality and early regression of the yolk sac are more specific than large size in predicting pregnancy loss 4
• A normal-appearing yolk sac of appropriate size is reassuring for pregnancy viability 1, 4

Recommended Management

Immediate Steps

• Continue routine first trimester combined screening with maternal serum markers (PAPP-A and free beta-hCG) if not already completed 1
• Document crown-rump length to ensure proper gestational age assessment and NT interpretation 1
• Confirm presence of embryonic cardiac activity, which should be evident in any embryo with measurable crown-rump length 1

Follow-Up Surveillance

• Standard second trimester anatomic survey at 18-22 weeks remains essential regardless of normal first trimester findings 1
• No additional specialized testing (fetal echocardiography, Doppler studies, or early anatomic survey) is indicated based on these normal measurements 1
• Routine prenatal care without intensified surveillance is appropriate 1

Key Clinical Pearls

Important caveats to remember:

• NT measurement quality depends on proper technique and operator experience - ensure the measurement was obtained according to standardized protocols 1, 3
• Normal first trimester findings do not replace the need for comprehensive second trimester anatomic evaluation, as only 50% of major fetal anomalies can be detected before 14 weeks 1
• If maternal age or other risk factors warrant genetic counseling, these normal ultrasound findings should inform but not replace individualized risk assessment 1