先天性疾病(congenital disorder)
在初生时就已经存在的各种结构畸形（如闭锁、发育不全）、功能缺损（如囊性纤维样变性和苯丙酮尿症）和其他疾病。几乎都是由於遗传因素（遗传突变、染色体疾病），怀孕时环境的影响（如风疹或其他母体的因素，曝露在毒素或辐射线中）造成的。妊娠的前八周以内，胚胎（包括其器官和组织）都已基本发育成形。故习惯上把八周之前的妊娠物称为「胚胎」，称八周以後的妊娠物为「胎儿」。大脑、眼、内耳等器官的成形分化是在胎儿期完成的，其畸形表现当然也只能在此期成形，但其畸形基础却是在胚胎期完成的。有的先天性疾病纯粹是孟德尔定律显性和隐性遗传造成的。有的则牵涉到复杂的基因问题。染色体疾病很少见，因为很少能存活下来的。每一千名初生儿中可能有三十名患有严重缺陷。不同的种族群其先天性疾病常有很大的差异。参阅birth defect和Down syndrome。
English version：
congenital disorder
Structural abnormality (e.g., atresia, agenesis), functional problem (e.g., cystic fibrosis, phenylketonuria), or disease present at birth. Almost all are due to genetic factors (inherited or spontaneous mutations, chromosomal disorders), environmental influences during pregnancy (rubella or other maternal factors, exposure to toxins or radiation), or both. The most sensitive period is the first eight weeks after conception, during which time the human embryo is essentially formed. Major congenital malformations form during this period. Some inherited disorders result from simple Mendelian dominance or recessiveness. Others may involve multiple genes. Chromosomal disorders are rare, because few affected fetuses survive to be born. Environmental influences may affect only one of a pair of identical twins. At least 30 significant defects probably occur per thousand births. Incidence of specific defects varies widely in different racial groups. See also birth defect, Down's syndrome.