Your best strategy is to get your employer’s attention focused on why they should promote you into a more senior role – and keep it there!
Ideally, only introduce the question of reducing your hours after you’ve demonstrated your ability to do a more senior job for the organisation and made all the key managers aware of your desire for promotion within the year. People tend to take more notice of the messages they hear first, especially if the message involves making changes (both yours do).
You could start off sowing the seed that you’re worthy of promotion and can do much of the work at home (the more convenient workplace for you) by volunteering to do some “next job up” thinking / planning / writing tasks without deadlines right now, before you’ve come back from maternity leave. If you’ve already proved you do a more demanding job better when the employer allows you to work at home, it’s difficult for management then to argue such arrangements will be impossible after your return to the workplace.
If there are opportunities to “tweak” the job so that it suits your new family circumstances better (eg more report-writing, fewer face to face meetings), then think about the “value to the employer” of the changes you’d like to make and present them accordingly. You might initiate video-conferencing, for example (web cams cost about £40) – most employers and workers are wary of trying it at the start but soon come to like the savings in time, money and hassle.
Do you have other colleagues who’d like a different pattern of working (eg perhaps someone who’s desperate to do another day’s work a week or to work more from home)? If so, think whether it would be “safe” and beneficial to team up and jointly suggest your preferred changes – sometimes there’s strength in numbers.
Good luck whatever you decide to do.