You have to learn to pronounce each word with Chinese too.
马 is mǎ, meaning horse. It's a picture of a horse, with 3000 years of adaptation.
女 means woman / female. It's a picture of a woman.
Put them together and 妈 means "the female thing that sounds like mǎ", which is mā (a different tone), meaning mother.
(I'm very much a beginner, but my teacher claims ~70% of characters follow this pattern.)
马, for example, is the simplified form of 馬, which is traceable back through the millenia to a pretty recognizable picture of a horse back around 1300 BCE or so. (http://en.wiktionary.org/wiki/%E9%A6%AC#Etymology)
Same with 女, which originally was a drawing of a stick-figure woman with breasts. (http://en.wiktionary.org/wiki/%E5%A5%B3#Etymology)