Locatelli A, Piccoli M G, Vergani P, Mariani E, Ghidini A, Mariani S, Pezzullo J C
Divisione di Ostetricia e Ginecologia, Istituto di Scienze Biomediche San Gerardo, Monza, Italy.
Am J Obstet Gynecol. 2000 Jan;182(1 Pt 1):192-7. doi: 10.1016/s0002-9378(00)70512-6.
Nuchal fold thickness is the best ultrasonographic predictor of fetal trisomy 21. However, the risk assigned on the basis of the commonly used threshold of nuchal fold thickness >/=6 mm does not take into consideration the significant associations between nuchal fold thickness and gestational age and between maternal age and Down syndrome. We propose a new method of calculating Down syndrome probability that takes into account both gestational age at examination and previously assessed probability of Down syndrome.
Nuchal fold thickness was measured at ultrasonographic examination at 14 to 22 weeks' gestation without previous knowledge of the fetal karyotype. Nuchal cystic hygromas were excluded from analysis. Statistical analyses included correlation, logistic regression to control for other ultrasonographic predictors of trisomy 21 and for maternal age, receiver operating characteristic curve, and likelihood ratios (defined as the ratio of the sensitivity to the false-positive rate). P <.05 was considered significant.
Mean gestational age at ultrasonography was 16.9 weeks' gestation (range, 14-22 weeks' gestation). Mean (+/-SD) nuchal fold thickness in fetuses with trisomy 21 (4.7 +/- 1.6 mm; n = 29) was greater than in euploid fetuses (3.2 +/- 0.9; n = 780; P <.001). Logistic regression analysis established that nuchal fold thickness was a significant predictor of trisomy 21 independent both of the other ultrasonographic markers and of maternal age (P <.001). Regression analysis showed that nuchal fold thickness was significantly correlated with gestational age among both fetuses with trisomy 21 and euploid fetuses and that the regression line of fetuses with trisomy 21 had a slope similar to that of euploid fetuses. The difference between observed and expected nuchal fold thicknesses on the basis of the biparietal diameter (as a function of gestational age) was used to obviate the confounding effect of gestational age. Differences between observed and expected nuchal fold thicknesses were then used to calculate likelihood ratios. These likelihood ratios could then be multiplied by the individual prior probability to obtain a patient-specific Down syndrome probability.
Nuchal fold thickness is correlated with gestational age in both euploid fetuses and fetuses with Down syndrome. Use of the difference between observed and expected nuchal fold thicknesses to determine likelihood ratios allows the calculation of individual posterior probabilities of Down syndrome that take into consideration both gestational age and maternal age.
颈部透明带厚度是胎儿21三体综合征最佳的超声预测指标。然而，基于常用的颈部透明带厚度阈值≥6mm所确定的风险，未考虑颈部透明带厚度与孕周以及孕妇年龄与唐氏综合征之间的显著关联。我们提出一种计算唐氏综合征概率的新方法，该方法同时考虑检查时的孕周和先前评估的唐氏综合征概率。
在妊娠14至22周进行超声检查时测量颈部透明带厚度，事先不了解胎儿核型。分析中排除了颈部囊性水瘤。统计分析包括相关性分析、用于控制21三体综合征其他超声预测指标及孕妇年龄的逻辑回归分析、受试者工作特征曲线分析以及似然比（定义为灵敏度与假阳性率之比）。P<0.05被认为具有统计学意义。
超声检查时的平均孕周为16.9周（范围14 - 22周）。21三体胎儿的平均（±标准差）颈部透明带厚度（4.7±1.6mm；n = 29）大于正常核型胎儿（3.2±0.9；n = 780；P<0.001）。逻辑回归分析表明，颈部透明带厚度是21三体综合征的一个显著预测指标，独立于其他超声标志物和孕妇年龄（P<0.001）。回归分析显示，21三体胎儿和正常核型胎儿的颈部透明带厚度均与孕周显著相关，且21三体胎儿的回归线斜率与正常核型胎儿相似。基于双顶径（作为孕周的函数）计算观察到的和预期的颈部透明带厚度之间的差异，以消除孕周的混杂效应。然后使用观察到的和预期的颈部透明带厚度之间的差异来计算似然比。这些似然比随后可乘以个体先验概率，以获得患者特异性的唐氏综合征概率。
正常核型胎儿和唐氏综合征胎儿的颈部透明带厚度均与孕周相关。利用观察到的和预期的颈部透明带厚度之间的差异来确定似然比，能够计算出同时考虑孕周和孕妇年龄的个体唐氏综合征后验概率。
