胞 (bāo) is a bound morpheme (b.f.) that cannot stand alone as a word. It appears in compounds related to 'born of the same womb' — most commonly in 同胞 (tóngbāo, 'compatriot') and 双胞胎 (shuāngbāotāi, 'twins'). You also see it in 细胞 (xìbāo, 'cell', literally 'small womb'). Knowing 胞 helps you decode these high-frequency words.