Genetic and Sonographic Insights into First-Trimester Fetal Cystic Hygroma: A Retrospective 30-Year Analysis Using 3D/4D Ultrasound and Cytogenetic Evaluation in Croatia (1993–2023)

Background/Objectives: Cystic hygroma is a congenital lymphatic malformation often identified during early pregnancy and frequently associated with chromosomal abnormalities and adverse outcomes. We aimed to appraise the genetic and clinical characteristics of fetuses diagnosed with cystic hygroma in the first/early second trimester, assess the resolution patterns in chromosomally normal cases, and provide insights into prognosis—based on data collected over a 30-year period. Methods: A retrospective cohort study was conducted on 405 consecutive fetuses diagnosed with nuchal cystic hygroma between 8.0 and 14.0 weeks of gestation from 1993 to 2023 at two tertiary care centers. Diagnoses were established using high-resolution transabdominal and transvaginal 3D/4D ultrasonography. All cases underwent prenatal cytogenetic analysis, including karyotyping. Fetuses with a normal karyotype were observed through serial ultrasounds through the remainder of the pregnancy to verify the eventual resolution of hygromas. Both descriptive and inferential statistical methods were used, with p < 0.05 as a cut-off (two-tailed). Results: Of the 405 fetuses, 210 (51.9%) had chromosomal abnormalities, most commonly trisomy 21, while 195 (48.1%) had a normal karyotype. A significantly higher frequency of trisomy 21 was observed compared to other identified chromosomal abnormalities (p < 0.001). In the chromosomally normal group, 85 (43.6%) showed spontaneous resolution of the hygroma within four weeks, and these pregnancies resulted in phenotypically normal live births. Septated hygromas were significantly more frequent in the abnormal karyotype group (71.4%). Conclusions: The finding and diagnosis of cystic hygroma in first trimester and early second-trimester pregnancy represent a strong predictor of chromosomal aneuploidy and warrant comprehensive prenatal genetic testing and close follow-up. However, in the absence of genetic abnormalities and additional malformations, spontaneous resolution is common, and neonatal outcomes are generally favorable. Health systems should provide equitable access to genetic testing and fetal imaging to support accurate diagnosis and informed decisions.