As far as the causes of pimple during pregnancy goes, there are many different reasons.
Literally anyone can get acne at any time in their lives and at any age, gender, race, or location.
Acne doesn't discriminate based on any of the above criteria, including and especially women who are pregnant.
Pregnant women have a lot going on inside of their body's, including fluctuating hormone levels, which is the main cause of acne at its root.
When hormone levels are out of whack within the body, excessive amounts of oils are produced and get into the pores of the skin becoming clogged, producing what is known as acne or pimples.
Usually shortly after birth women no longer experience problems with acne breakouts or pimples, but in some cases the acne does continue, maybe for different reasons.
Other reasons a woman could continue to have problems with pimples is because her diet isn't as healthy or nutritious as it could be.
Pregnant women more so than any other woman need to take care of themselves and get the proper amounts of vitamins and protein in their diet or else there could be complications with pregnancy and other parts of the body including the skin.
The pimples and zits that pregnant women may experience are usually the worst within the first 5 or 6 months of the pregnancy because that is when the hormone levels fluctuate the most in the body.
As stated above, the acne breakouts usually stop a couple weeks after the baby is born, but sometimes they continue in which case the woman should see a dermatologist right away.
Not treating your acne while you are pregnant can result in complications with the baby which can easily be avoided by treating it when it appears and not putting it off till a later date.