Hi,
I am a scientist. Technically my contract says 37 1/2 hours per week, but of course we are supposed to be professionals and as such before DD was born I did way more than 40hours per week. Since DD was born I tend to stick to my hours. This does mean that I don't have the same number of responsibilities that I had previously, but this partly as when I was on maternity leave my boss left and when I came back I got a new one who wasn't such a slave driver. I do worry that my career is affected sometimes, but then I think that I also want a better life balance and if that means that I don't progress as much or get the same number of opportunities to attend conferences etc, then so be it. I am lucky that my employer is quite childcare friendly (though we are about to be taken over so that may change).
You can do it if you have good childcare, I have friends working for large London law firms, but they don't see their kids Monday-Friday and that's a decision they had to make. But they do make a huge effort spending quality time on the weekend and having plenty of vacation. It depends what you want really. I also had friends who quit their jobs on maternity leave as their employer told them they would have to come back doing their old hours with no option to reduce. In fact one of these also worked for a construction firm but here in the UK, she was told if the client needed her there then they needed her there and she would need to accommodate that and couldn't work to a set of defined hours.
I think you just need to think about what you want and if you don't mind giving up some responsibility. It may not mean a step down, but perhaps a lack of progression for a few years on the track you were previously on. And maybe consider other employers as well who may have more family freindly policies. Good luck!