Aside from collagen, there are several vitamins that may help with skin elasticity. These are vitamin C, vitamin A, and vitamin E. All three of these antioxidants are useful for protecting your cells from damage (source).
Although they won’t directly contribute to tightening loose skin, if these vitamins are taken during pregnancy, they can help increase elasticity, which can help you avoid excess loose skin after your baby is born.
The best way to absorb vitamins is through food. Look for foods high in antioxidants, including vitamins C, A, and E to help prevent loose skin. Some good choices are citrus fruits, melon, berries, sweet potato, leafy greens, carrots, avocado, cocoa, nuts, and seeds.