Previous research of in vitro placental perfusion indicated that the transfer rate of dexmedetomidine through the placenta to foetus was 0.77, and the other study indicated that the rate of placental transfer of dexmedetomidine in cesarean section operation under general anesthesia was 0.76. It's indicated that dexmedetomidine can also easily pass through the placental barrier like other anaesthetic drugs. However, the placental transfer rate of dexmedetomidine is much lower than that of clonidine(0.85) and that of remifentanil(0.88), which may be caused by dexmedetomidine being more fat-soluble and easier to be retained in the placenta.