Hi Saffy, we are just ahead of you, with DS at around 20 weeks now. I think you will find that the wonderweek your bubs is going through does go for about three-four weeks, with two phases in it (I only have the Wonder Week app which tells you a bit about each wonder week, not the book). Sleeping for my DS has been really variable over the past weeks, but having said that, he never really has been in any sort of pattern (definitely no sleeping through in this house!!). My DS also has not been feeding well due to maximum distraction from DD (he can't take his eyes off her), so any additional feeds at night I have been attributing to lack of/short day feeds. This is the age when distraction really kicks in- has your DD been feeding okay during the day?
I can report that the last couple of days have been very sunny for DS- lots and lots of smiling, and wanting to sleep a lot during both the day and night so hopefully your sunny week is just around the corner!