Fraternal – nonidentical – twins develop from two separate fertilized eggs. By 6 weeks, their hearts are beating, and at 8 weeks all of their major organs start developing. The twins’ faces take shape, with closed eyelids, at 12 weeks, and at 16 weeks their sex organs are apparent.
In the weeks following, the babies continue to grow and learn to control their body temperature. The average birth week for twins is 36 weeks, and by then all body systems are functioning.
Around 26 weeks of gestation twins do slow down a bit in growth in comparison to singletons, as their environment gets pretty crowded! Nonidentical twins like these (also known as fraternal or dizygotic twins) are the most common.