I mean it's pretty clear she thought they were seriously dating, considering he'd been telling her they'd get married one day. Like allow me to be a little rude but if you read the chapter, you'll notice she says this on the first page.
Also she kept the baby because she's pretty desperate for family, like given how she tells us her parents divorced and built their own new families when she was young, and that she's been alone for most of her life, yeah keeping the baby and having a family of her own was clearly very important to Chinatsu.