Answer: There is a couple of reasons as to why not, One is the hormones while pregnant can affect your skin and the retention of the ink 

One of the main concern with getting a tattoo during pregnancy is the risk of contracting an infection, such as Hepatitis B and HIV. Although the risk is small, it is recommended that you wait to get a tattoo until after your baby is born.

And we don’t really know how tattoo inks/numbing effects a developing baby. Small amounts of chemicals that might be harmless to an adult can have a much bigger impact on a tiny fetus and just not worth the risk