双胞胎（英语：twins），别称孪生、孖生、双生，指胎生动物一次怀胎里同时生下两个个体的情况。

人类生育的通常情况里，女性每一次月经周期，卵巢只会释放一粒卵子，受精后只会发育成一个胚胎，出生时自然也就一胎生一个。但少数特别情况下可能同时有两个甚至更多个胚胎发育，称之为孪生、孖生或双生。

双胞胎的性别搭配有三种：男双，女双及混双；其中男女各一的混双，东方传统称为「龙凤胎」。

除了双胞胎，三胞胎、四胞胎甚至更多胞胎也可能自然产生，当然胎数越多越奇特稀有。

Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic ("identical"), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ("fraternal"), meaning that they develop from two different eggs. In fraternal twins, each twin is fertilized by its own sperm cell.