Cultural differences can also be a theme. Since Hetalia is based on different countries, the characters may bring in their cultural ways of handling pregnancy. For instance, a character representing a more family - oriented culture might be more involved in traditional pre - birth rituals. And family dynamics is also a theme. How the new baby will change the existing relationships among the characters and the reader.
Well, often there's the theme of growth. The pregnant reader grows not just physically but also emotionally during the pregnancy. The Hetalia characters also grow in their understanding of family and responsibility. There might be challenges along the way, like dealing with morning sickness or financial concerns for the baby's future, which are also common elements in these fics.