@adachika192 わたしも苦手です。ワーママ、という言葉はまだ良いんですけど。シンママは。。。
Good to know your interest, Trisha-san!
The only term came to my mind was 「未婚の母」(mikon no haha), meaning "a mother not yet married", which clearly is a way short of denoting a variety of situations for being a single female parent.
「母子家庭」(boshi katei), a family with a mother and her child(ren), may come next, which also failed for not meaning the mother herself... 😑
Without being an expert, I could easily imagine it has something to do with the moral about 'ideal of family' in Japan.