Ladies you absolutely have to use factor 50 every day! And reapply on your face every 2-3 hours I'm afraid. I get it with every pregnancy and although it fades after pregnancy if you darken it further with tanning it can take years go, or involve a lot of expensive laser like I had to have to get it fairly under control. Pigmentation is no joke that's for sure, some may be hyper pigmentation, some may be post inflammatory, and some may be melesama, and hormones. The prevention of factor 50, preferably sink or mineral sunscreen, is the only way, the cure is post baby to keep on with the factor 50 unfortunately (because once you've had it you are even more susceptible, so don't ever go trying to use hair removal cream on your upper lip or face unless you want much more permanent brown marks!)
Only perescription strength retinol, hydroquinone bleaching 4 percent or higher (also only prescription) or laser treatment actually fades these marks if you don't wear a factor every day, even on a very cloudy one.
It will fade a lot when you've had the baby, it's very much linked to hormones, but our skin does change when pregnant and becomes more sensitive to environmental factors so definitely invest in some decent mineral or a physical sun block (the chemical ones contain really unpleasant ingredients).
I can link a few brilliant options next, one is less 'natural' i.e not a physical barrier but one that sinks in and creates a reaction with the skin (like all the ones on the shelf at boots/superdrug ect) but was recommended by my dermatologist who treats many patients who have had skin cancer so I trust her judgement