During pregnancy a women goes through drastic body changes relating to blood circulation, blood pressure, hormonal fluctuations, weight gain and more and all of which put the immune system under huge strain.
Pregnant woman could also faint from high blood pressure. Blood can be thinner resulting in more bleeding during treatment and prevent the pigment from staying in the skin. Hormones can also affect the colour of the pigment. There is also the risk of post procedure infection which could harm the unborn child.
So the answer is no, I do not perform semi-permanent treatment on anybody that is pregnant or breastfeeding.
I do recommend a minimum of 3 months after giving birth to have any treatments done, this is to ensure the hormone levels have settled back down.