Corticoids are well known for causing wide and deep stretch marks. You would have probably developed stretch marks anyway as a result of your pregnancy (about 90 percent of women develop stretch marks during pregnancy) but corticoids likely aggravated the depth and width of your stretch marks. Corticoids weaken the connective tissue in the skin and make the skin vulnerable when stretched.
In your case, I would recommend a NARROW 1.5 mm dermaroller. Whenever you have time (certainly not easy to find much time when looking after a 2 year old ;-) roll densely and thoroughly over several individual stretch marks and massage the A-Ret (Tretinon) gel into it (it will likely sting). Meaning you will select one stretch mark, roll it thoroughly, massage Tretinoin gel into it, select another stretch mark and do the same. It is up to you how many stretch marks you roll in one session but do not do your whole belly in one go. Better to do it in parts. Do not dermaroll the same stretch mark more than once a month. Start for example on the left side of your tummy and roll every day five marks (every day new ones) until you will have done all of them. Do this every month
After three months (each stretch mark has then been rolled three times), give your skin a one-month break and then again roll for three months, again one month break etc.
When you roll an individual stretch mark with the narrow dermaroller, you will try to concentrate on, and target the stretch mark but the dermaroller will also "hit" the skin surrounding the mark, which is normal and OK.