Hi Monnalisa,
That still seems very broad, and I wonder if that will help you get the answers you want. Patterns might be different by subculture, age, position in society, geographical location within Japan, etc. I think you would help yourself to find a more homogenous group (for example; couples in Tokyo between 30 and 40 that have been together five years and have 0/1/at least kids. IUnless you want to compare data within this group, but then you really need a lot of respondents.
I think your best bet to get this spread out, is to post it on fora that are often frequented by Japanese women. Maybe people here can point you towards those sites. I unfortunately can't.