When it feels uncomfortable (ie you find it hard to breathe, feel dizzy or light-headed, etc - ie signs that the weight of your uterus is compressing your organs, inferior vena cava etc). Your body will let you know, so if you're happy on your back, don't worry about it - you won't hurt yourself or the baby! I'm 29 weeks and still lie on my back a lot of the time.
I don't know the exact timescale but I think you will know naturally when it is no longer comfortable. Nothing bad is going to happen from you turning onto your back in the middle of the night at this stage. Eventually, you will feel the weight of the bump so much more that after a couple of times of trying to sleep in your usual way it will no longer be possible. If you do go to sleep on your back you might want to try and see if you can start from another position, but you soon won't have the option to do otherwise anyway!
Is it your first? There comes a point in your pregnancy when you are so massive that just turning in bed will become difficult and uncomfortable. You will be able to tell it is putting pressure on your back and spine at that point. If you do wake up on your back then, there isn't much you can do about it anyway but you'll probably notice you're not comfortable in that position and switch (or attempt to!) to your side to go back to sleep.
I'm 16 weeks and still sleeping in my usual position on my stomach. I occasionally wake up on my back and it still feels OK. When I can no longer be comfortable lying on my front it will be the time to try to sleep on my side!
I'm 32 weeks & every single night no matter how hard I try I find myself sleeping on my back. It makes me feel horribly guilty that i might be hurting baby but after a day spent running round after a toddler I just sleep in any position I can
I'm pretty sure you wake up when it gets dangerous for your body/baby. I keep waking in the night from getting sore hips on my side, so I often fall asleep again on my back, but then I wake up with pins and needles down half my hand, or just from feeling uncomfortable (like I'm not getting enough air/blood flow through my body).
Though I'm nearly 21 weeks so don't have a massive bump yet!
I am 36 weeks and I still sleep on my back, I do flit from side to side and use a full length body pillow for spd, to be honest it hurts sleeping on my side. sleep is sleep so however comfortable you are enjoy x
I'm 26 weeks and have slept on my left side since 16 weeks because I started to get problems with caval compression then. I still fall onto my back at times but this always wakes me as I feel dizzy and ill. Same as last pregnancy for me.
It's not necessarily dangerous to sleep on your back. The advice not to came from a study of pregnant women when it was observed that they tended to become breathless or have faster heartbeats whilst lying on their backs. However, all of these women were in hospital for various pregnancy problems, so they're not an accurate reflection of the general pregnant population! If your body doesn't like it, you will instinctively either change position in your sleep or wake up feeling uncomfortable (and then change position). If you feel fine lying on your back, you won't be doing any damage. It is true that lying on your left side in the last few weeks encourages the baby to get into the optimal position for labour - but if you find that uncomfortable, sleep however you want to and spend a bit of time on your hands and knees when you're awake instead, which has the same effect.
Thanks OP & lurcher, I was going to ask a similar question. 18wks w DC2 and just had the "advice" re left side sleeping pop up on a pregnancy app. Seemed to remember last time reading something explaining the advice was out of date, as most comfortable on my back or my right (subject to DH's mouth breathing on left).
I'm 34+5 and just spent most of last night on my back, because like lauraellajane my hips get very painful when lying on my side for any length of time. Actually it's not the problem with the vena cava which is affecting me on these occasions, it's the weight of the baby pressing on my left kidney. Bloody impossible to breathe properly without causing pain!
Thanks for all the responses! I'm not usually a back sleeper, but during pregnancy I seem to find myself waking up on my back a lot, so glad to know that it's fine as long as it feels comfortable (common sense, right?).