Why does sleeping position matter in the second trimester?

In the second trimester your belly grows noticeably and your sleep starts to change too. It is a good time to get used to sleeping on your side, because after 28 weeks you are advised to go to sleep on your side.

What happens when you lie on your back?

As your womb grows, it gets heavier. When you lie on your back, it can press on the large vein that carries blood from the lower half of your body back to your heart (the inferior vena cava). This pressure reduces the blood returning to your heart; your blood pressure can drop for a short time and you may feel dizzy. That is why, later in pregnancy, you are advised to avoid lying flat on your back for long periods.

Why go to sleep on your side after 28 weeks?

Research shows that going to sleep on your back after 28 weeks is linked to a higher risk of stillbirth. The safest position to fall asleep in is on your side, left or right. The exact reason is not known; the leading explanation is that the weight of the baby and womb can press on the main blood vessels that supply the womb and reduce the flow of blood and oxygen to the baby. Another possibility researchers are looking at is disturbed breathing during sleep, which is more common when lying on your back.

This advice is not only for nighttime sleep. Start every sleep on your side, including daytime naps and whenever you settle back to sleep after waking in the night.

Lying on your back can also make backache, constipation and hemorrhoids worse. Getting used to a new position takes time; if you start practicing side sleeping in the second trimester, it will feel much more natural by the time you reach 28 weeks.

Left or right?

Either side is fine. A large UK study on this topic found no difference in risk between the left and right side. Many doctors prefer the left side; you can lie on whichever side is comfortable for you and switch between sides during the night. If you have heartburn, lying on your left side may ease it.

“What matters is not the position you wake up in, but the position you fall asleep in.”

If you wake up on your back, don't worry; changing position during sleep is completely normal. Simply roll onto your side and go back to sleep. The position you fall asleep in is usually the one you stay in the longest during the night, so how you fall asleep is what matters most.

Tips that make side sleeping easier

• Keep your knees and legs slightly bent, and put a pillow between your legs.
• Support your belly from underneath with a small pillow, or try a full-length pregnancy pillow.
• Place a pillow behind your back; it makes it harder to roll onto your back in your sleep.
• If you have heartburn, raise your head and shoulders together, slightly.
• If you feel tired, nap when you can, and use the same pillow setup to lie on your side for daytime naps too.
• When you exercise, avoid moves that keep you lying flat on your back for long periods.

When should you contact your doctor?

If you notice your baby moving less or a change in their usual pattern of movements, go to labor and delivery right away — don't wait to count kicks or reach 10 movements.

If dizziness or feeling faint that starts when you lie on your back does not ease when you turn onto your side, tell your doctor about that too.