If you want to use the serum as well, then apply the serum before the lotion.
I've only used the lotion during my last pregnancy, and was applying it over old stretchmarks (from puberty) and new ones (from pregnancy). I purchased two of these Stretch Marks Double Action tubes total, and my stretch marks are GONE. The ones on my belly waited to fade until baby came out but the skin wasn't wrinkled after delivery, just smooth with a couple colored lines that are fading now. I started use at week 24 of pregnancy and have just delivered.
I highly recommend this product, since no other line I've found delivers results AND is safe for baby (which means it's probably pretty safe for you, too, and not a sneaky carcinogen). I bought Strivectin and can't even use it because I'm breast feeding, what a waste of money.