High priority traffic of Wireless Body Area Networks (WBANs) requires high reliability and low-latency communications with consideration due to energy efficiency and network lifetime. However, the wireless links of WBAN are very unreliable due to the high mobility of the body and the ultra-low transmission power of nodes, which make meeting these requirements a significant challenge. In order to overcome the low quality wireless links, the IEEE 802.15.6 standard for WBANs provides the two-hop relay option. Some relay schemes for WBANs have been proposed in recent literature but do not consider all the above performance requirements. An opportunistic relay scheme exploiting channel coherence time is proposed here to meet these requirements. The theoretical analysis and simulation results of the proposed scheme compared with the one-hop transmission show that it can greatly improve the network reliability and reduce transmission delay at little energy cost in a deep fading channel, and also prolong network lifetime.