When skin is stretched for a long period of time it looses its elasticity which leads to stretch marks. The key is preventetion, especially during pregnancy. Essential oils have very strong regenerative abilties and can help a lot by improving your skin apperance. Rich carrier oils such as calendula, avocado, argan, with additions of rosehip seed are all of essence here.
Most important essential oils for this are lavender, frankincense, geranium and neroli. In pregnancy or weight gain/loss preventative measures are better than cure and will judicious use of the aromatherapy oils you can achieve great results.
You can find stretch mark oil at the bottom of this page.