You generally want to stop lying flat on your back around the middle of pregnancy, roughly after 20 weeks or by the end of the second trimester , and switch to mostly side‑lying (especially the left side).

Why you’re told to avoid your back

  • After about 20–28 weeks, your growing uterus can press on a large vein called the inferior vena cava when you lie flat on your back.
  • That pressure can reduce blood flow back to your heart and to the placenta, which may cause:
    • Dizziness or light‑headedness
    • Feeling faint, short of breath, or nauseous
    • A drop in blood pressure and potentially less oxygen/nutrients getting to the baby.

Some UK and international guidelines now advise avoiding going to sleep on your back in the second and third trimesters , and especially in the third trimester (after 28 weeks).

What research says

  • Several studies and reviews have suggested that regular back‑sleeping late in pregnancy may increase the risk of stillbirth , probably because of reduced blood and oxygen flow to the baby.
  • However, more recent work has also found that back‑sleeping up to about 30 weeks did not clearly increase risks in all populations, so some experts are slightly more relaxed and say to avoid it mainly in the later third trimester.

Because the risk (even if small) is potentially serious and the change is simple, many clinicians still recommend:

Try to fall asleep on your side from about 20 weeks onward, and especially after 28 weeks.

What if you wake up on your back?

You don’t need to panic if you wake up and find yourself on your back.

  • Studies and expert groups emphasize that the position you go to sleep in seems more important than the position you drift into briefly at night.
  • If you wake up on your back:
    1. Roll onto your side (left or right).
    2. Use pillows behind your back or between your knees to keep you there if you tend to roll.

Many pregnant people on forums also report that their bodies “tell” them it’s time to move—lying flat suddenly feels uncomfortable or causes dizziness or pressure, which makes them roll onto a side.

Safer, more comfortable positions

Most experts suggest side‑sleeping as the go‑to position after the first trimester.

Helpful tricks:

  • Sleep mostly on your left side : this often gives the best blood flow to uterus and kidneys.
  • Use a pregnancy pillow or extra pillows:
    • Between your knees
    • Under your belly
    • Behind your back to stop you rolling flat.
  • If you miss your back:
    • Try a propped‑up/reclining position with several pillows so you’re at an incline instead of completely flat; that lessens pressure on the vena cava.

Quick FAQ style answers

  • When exactly should I stop lying flat on my back?
    • Aim to stop routinely lying totally flat on your back from about 20 weeks , and definitely by the third trimester (28+ weeks).
  • Is it safe earlier on?
    • In the first trimester , back‑sleeping is generally considered safe because the uterus is still small.
  • What if my doctor says it’s fine?
    • Some OBs and midwives tell patients they can keep any position as long as they feel well, noting that symptoms like dizziness are a warning sign.
* If your provider has different advice, follow them—they know your specific pregnancy and health history.

Bottom line

  • Try to transition away from lying flat on your back around 20 weeks and especially avoid going to sleep that way after 28 weeks.
  • Prioritize side‑sleeping , especially the left side, and use pillows or a slight recline to stay comfortable.
  • If you wake up on your back, just roll over; occasional short periods on your back are unlikely to cause harm, but discuss any worries, dizziness, or breathlessness with your own clinician.

Information gathered from public forums or data available on the internet and portrayed here.