This observational study aims to recruit pregnant women between 18 to 24 weeks of gestation to investigate the relationship between amniotic membrane thickness and fetal chromosomal abnormalities. The primary objectives are to establish whether a correlation exists between the measured thickness of the amniotic membrane and the presence of chromosomal abnormalities in the fetus, and to determine a cutoff value for amniotic membrane thickness that could indicate an increased risk of such abnormalities. Additionally, the study seeks to assess whether the inclusion of amniotic membrane thickness as a biomarker can enhance the detection rate of non-invasive prenatal testing (NIPT) and nuchal translucency (NT) for chromosomal abnormalities.

Inclusion Criteria:

1. Singleton pregnant women between 18-24 weeks of gestation.

2. Participants with high-risk indications for prenatal fetal chromosomal abnormalities, including:

   • Nuchal translucency (NT) ≥ 3 mm.
   • High-risk results from non-invasive prenatal testing (NIPT) using cell-free fetal DNA from peripheral maternal blood.
   • Ultrasound-detected fetal abnormalities.
   • Other indicators include adverse birth histories, parental chromosomal abnormalities, familial diseases, and a history of thyroid cancer post-surgery.

3. Gestational age and estimated due date were confirmed by the first day of the last menstrual period and adjusted using fetal crown-rump length measured during the first-trimester ultrasound scan.

4. Participants who provided written informed consent for amniocentesis.

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Exclusion Criteria:

1. Women with multiple pregnancies or higher-order births.
2. Previous chorionic villus sampling or amniocentesis in the current pregnancy.
3. Gestational age at the time of amniocentesis greater than 25 weeks or less than 18 weeks.
4. Presence of amniotic band syndrome.
5. The presence of uterine anomalies or conditions may impact ultrasound measurements' reliability.
6. Any medical condition or obstetric complication that, in the opinion of the investigators, might pose a risk to the participant or interfere with the study outcomes.

The study population included pregnant women referred to the Center for Prenatal Diagnosis and Fetal Medicine at Tianjin Central Hospital of Obstetrics and Gynecology. These participants were identified based on clinical indicators of elevated risk for fetal chromosomal abnormalities. The cohort represented a diverse age range of expectant mothers, predominantly from Tianjin's urban and suburban regions in China.

Focused on singleton pregnancies, the study minimizes variables linked to the complexities of multiple gestations. The women were chiefly in their second trimester, a pivotal time for fetal development and diagnostic evaluation.

A considerable portion of the study participants had received high-risk NIPT results or demonstrated ultrasound markers suggesting potential chromosomal irregularities. This selection was vital for evaluating FM thickness as a diagnostic tool for chromosomal anomalies.