Sleeping position..?

Just • 2 girls & baby girl no3 on her way 😘
So I remember with my DD (back in 2010/11) there was somthing about sleeping positions when pregnant. I can remeber the not to sleep on back after half way point of 20 weeks but im sure they was a side that's best to sleep on as well. 
Could anyone please remind me of these sleeping guidelines in regards to position. 
Thanks in advance.