Are these stretch marks?
Stretch marks are common, but not universal. As well they vary in location and severity. Their onset can be at any stage of pregnancy though they are more common as the pregnancy advances. They are more of an issue in your first ongoing pregnancy, often not getting worse in subsequent pregnancies. They are worse with overstretching of the skin such as occurs with twins and with very large babies. They are due to tearing of the deeper layers of the skin.
What can be done?
To my knowledge they can’t be prevented.
There are many creams, etc on the market but to my knowledge there is no definite scientific evidence they make any difference! Sorry! If someone has reputable scientific information to the contrary please advise me.