Baby not babbling?

My daughter is 7.5 months and she doesn't babble. A couple of babies in our mums group have started saying mamama, dadada, bububub etc. And most of them are younger. My daughter is often very quite, she's a happy baby and she'll giggle and smile when people talk to her and she'll occasionally coo but nothing that sounds like babbling yet. We read her books almost every day, don't have the TV on often, speak to her like we're having a conversation. I'm a little worried that she should be babbling or at least 'talking' more by now.