I expressed at 4 weeks. It took a while for the pump to get any milk out though. I expressed after she had fed which also helps your supply. Just stick the pump on and go,don't worry of none comes out though. It can take a few goes to work, plus your body is still building up your supply at the moment. My LO takes bottle and breast fine, but it is recommended not to introduce a bottle until six weeks to avoid nipple confusion. I don't however believe in nipple confusion I think if a baby choses a bottle it is showing a preference rather than being confused. However you need to be aware that milk from a bottle is easier for baby to get than from a breast and so they can decide they prefer a bottle. Not all babies though prefer bottle mine prefers boob hands down.