The second trimester begins with the start of the 13th week, when the twins are now referred to as fetuses (as opposed to embryos during the first trimester). In the beginning stages of this 3 month period, the multiples' mouths are developing enough to practice sucking and swallowing reflexes, and the fingerprints are barely becoming visible. It is around this time that their kidneys and digestive systems take shape and begin the processes of elimination into the amniotic fluid. At around 2-4 ounces (and approximately 2-4 inches) each, they will also start hearing their first sounds and listening to every noise your body makes-from your heartbeat to your inevitable digestion. These sounds will serve to comfort them in the womb as they mature further. It is at this stage when the unpleasant side effects of early pregnancy (morning sickness) start to fade.
As they approach the middle of this trimester they will begin to start interacting with each other, pushing and bumping back and forth. Somewhere toward the middle of this period, a process called "quickening" occurs; this is the point at which mommy can feel the babies moving inside their belly. Some have described the feeling of fluttering wings in the belly. This happens at different times during all pregnancies-so there is no need to worry if you are one of the late ones. This is also the time that the sex organs are becoming fully developed, and the sex of the babies will usually be determinable by an upcoming ultrasound. You should probably decide at this point whether or not you want to know this information-so no one blurts it out at a doctor's visit.
At weeks 20-24, toward the end of this trimester, there are significant milestones occurring. The arms and legs become more fully developed, which will lead to more significant kicking and bumping in the belly. Eyelids have developed, and the eyes will blink for the first time, at by the end you will see amazing ultrasound pictures of hair, fingers, and even eyebrows (if you choose to look)!
The second trimester ends with a happy term-viability. This is the 24 week point that is considered the safe point for premature births to survive in the NICU (Neonatal Intensive Care Unit). From this point on, every day in the womb increases chances for survival, and exponentially reduces the long term effects of premature births.