TTTS usually develops during the period of peak placental growth, starting in week 16 and proceeding through week 25; after this point, the placenta's growth decelerates, essentially stopping just after week 30. While TTTS has occasionally been detected beyond this timepoint, it is thought that its occurrence beyond week 30 may be due to a placental embolism that upsets the flow balance of the shared connections between the babies. TTTS is potentially lethal to either or both twins, no matter when it is detected. However, when detected past week 25, emergency delivery may be considered to rescue the babies if the TTTS is severe.