Thought it was longer than a month. So was thinking about abortion or w/e since no milk yet. Although I believe a Woman would still start lactating if the hormones have shifted already anyway.
As per quick Google Search.
Milk starts producing around 4th month of pregnancy, but that's in low quantity for majority of women. Majority Milk production starts after giving birth for majority of women.
As for Breast Size increase that occur during pregnancy, that starts occuring around a month from impregnation.
So, timeline wise it all fits, aka., MC had impregnated her around a month or two ago, which fits the period that should match with Breast size increase, but it's a bit early for Lactation to occur.
As for the Abortion stuff, i don't think Prostitutes go for Abortion that much (atleast in these kind-off setting), as it badly affects their Body, which is basically their Product and Source of Income.
They would use Anti-Pregnacy medications or tricks to avoid getting pregnant. But if they do end up getting pregnant, they most likely go for full term with it and give birth, and depending on the type of person the mother is, either she takes care of the child or abandons it.
Plus, we don't really know how good abortion system is in that world.
Also, if she had gotten an abortion, then she wouldn't remain in "impregnation" status, and instead been on "normal" status, meaning MC should have impregnated her again and gained a level.
There is also a possibility that none of the Women impregnated by MC can abortion the child, as the pregnancy caused by MC is based on his Skill giving to him by a Godly being. So, basically the children can be said to have been conceived through a divine blessings, probably making them immune to abortion.
As mentioned before, MC was also born from a prostitute mother, and he was taken care by the store until his mother was alive (after she died, MC was kicked out).