单词crown-rump-length
网络: 顶臀长度；头臀长；冠臀长
The crown-rump length becomes less accurate after 12 weeks in determining gestational age because the fetus begins to curve.
孕12周后，因胎儿身体屈曲，以胎儿顶臀径推算孕龄已不够准确。