Sleep position becomes an important consideration for maternal and fetal well-being as a pregnancy progresses beyond the first trimester. The increasing size and weight of the growing uterus significantly change the body’s internal mechanics, which can affect circulation during rest. Adjusting the way one sleeps can help ensure optimal blood flow and comfort throughout the later stages of gestation.
Determining the Ideal Time to Change Sleep Position
The ideal time to transition to side sleeping is generally around the start of the mid-second trimester, or approximately 20 weeks of gestation. This timing serves as a sensible period to begin practicing a new sleep habit before the uterus becomes large enough to cause significant physiological changes. While it is safe to sleep in any position during the first trimester, the shift should be well-established by the time the third trimester begins.
The most evidence-backed period for making this change mandatory is from 28 weeks onward, as this marks the beginning of the late-pregnancy window where supine sleeping carries a higher risk. Research suggests that consistently settling to sleep on the side from this point can reduce the likelihood of complications associated with sleep position. The change is driven by the sheer mass of the uterus, which reaches a size at this stage where it can compress major vessels when a person lies flat on their back.
Understanding the Physiology of Left Side Sleep
The recommendation to sleep on the side, particularly the left side, stems from the body’s vascular anatomy. The Inferior Vena Cava (IVC) is the large vein responsible for returning deoxygenated blood from the lower body back to the heart. This vessel runs along the right side of the spine, making it vulnerable to compression when the pregnant person lies on their back.
When the heavy, pregnant uterus presses down on the IVC in the supine position, it restricts blood flow returning to the heart, a condition known as aortocaval compression. This restriction reduces the mother’s cardiac output and can lead to a drop in blood pressure, sometimes causing symptoms like dizziness or nausea. Crucially, this compression can also decrease blood flow through the aorta, which supplies the uterine arteries that carry oxygen and nutrients to the placenta.
Positioning the body on the left side shifts the weight of the uterus off the IVC, which sits to the right of the midline. This left lateral tilt ensures the vein remains open, maximizing venous return and maintaining optimal circulation for both the mother and the fetus. Maintaining this position supports kidney function, which can also help to reduce common pregnancy swelling in the hands, ankles, and feet.
Tips for Comfort and Staying on Your Side
Shifting to a side-sleeping position can be challenging, especially for those accustomed to sleeping on their back or stomach. To maintain the position throughout the night, use supportive devices like a full-body or wedge-shaped pregnancy pillow. These specialized pillows are designed to cradle the body, providing necessary support for the abdomen and back.
Using Pillows for Support
- Place one pillow between the bent knees to align the hips and relieve pressure on the lower back.
- Use another small pillow or wedge under the abdomen to provide a gentle lift and support the weight of the growing bump.
- Place a firm pillow or rolled-up blanket directly behind the back to act as a physical barrier against inadvertently rolling flat onto the supine position.
What About Sleeping on Your Back or Right Side?
Sleeping flat on the back is the position most strongly discouraged in late pregnancy, typically after 28 weeks. Studies have shown that going to sleep in the supine position is associated with an increased risk of late stillbirth, related to the compression of the IVC and aorta when the full weight of the uterus rests on these vessels.
While the left side is often cited as the preferred position for optimizing blood flow, current research indicates that sleeping on the right side is also acceptable in terms of stillbirth risk. The most important factor is avoiding the supine position, and either side-lying position achieves this goal. If a person wakes up momentarily on their back, they should simply roll back onto either side without undue alarm, as the position when settling to sleep is considered the most significant factor.