Boleh je at 13 weeks, sebab baby still kecil dan placenta belum fully developed yet, tapi as you progress nanti, better to shift ke side sleeping, especially after 20 weeks or so A few of us tried terus tidur telentang sampai later stages and eventually rasa uncomfortable je sendiri, so your body will tell you when to switch! If you're worried about safety or pusing-pusing, check with your doctor la, but honestly at this stage enjoy the comfort while you can because come third trimester, tidur positioning becomes proper tricky hehe.