The most important position to avoid during pregnancy is lying flat on your back, especially after 20 weeks. As your uterus grows, its weight can compress a major blood vessel called the inferior vena cava, reducing blood flow to both you and your baby. Beyond sleeping, certain exercise positions, standing habits, and sitting postures also matter as your pregnancy progresses.
Why Lying on Your Back Becomes Risky
The inferior vena cava is a large vein that runs along your spine, roughly at the level of your lower back. It carries blood from your legs and lower body back to your heart. When you’re upright or on your side, gravity keeps your uterus from pressing on this vein. But when you lie flat on your back, the full weight of your uterus settles directly onto it, squeezing the vein against your spine.
This compression slows the return of blood to your heart, which can cause a rapid drop in blood pressure. You may feel dizzy, nauseous, clammy, or pale. Your heart rate may spike as your body tries to compensate. This is sometimes called supine hypotensive syndrome, and it resolves quickly once you roll onto your side, which lifts the uterus off the vein.
The concern goes beyond your own discomfort. Reduced blood flow through the vena cava also limits blood reaching the placenta. A large meta-analysis published in eClinicalMedicine found that women who fell asleep on their backs in the third trimester had 2.6 times the odds of late stillbirth compared to women who fell asleep on their left side. The researchers estimated that if every woman in the third trimester settled to sleep on her side, late stillbirths could be reduced by roughly 6%.
How to Sleep Safely After 20 Weeks
Side sleeping is the recommended position from mid-pregnancy onward. The left side is often suggested because it keeps the uterus from pressing on the vena cava and may optimize blood flow to the placenta, but either side is fine. Placing a pillow between your knees and another behind your back can help you stay comfortable and prevent rolling onto your back during the night.
If you wake up on your back, don’t panic. The position you fall asleep in is the one you spend the most time in, so simply roll to your side and settle back to sleep. Many women find that their body naturally signals them to move before any real harm occurs, since the dizziness and nausea are hard to sleep through.
Stomach Sleeping and When It Stops Working
Sleeping on your stomach is safe in early pregnancy. The uterus is still tucked behind the pelvic bone, and there’s no meaningful pressure on the baby. Most women naturally stop sleeping on their stomachs somewhere in the second trimester simply because it becomes uncomfortable as the belly grows. Research on prone positioning during pregnancy shows that full stomach-lying is generally impractical beyond about 32 weeks due to the size of the uterus, and surgical procedures requiring face-down positioning are mostly limited to the first and second trimesters for this reason.
If you’re a natural stomach sleeper, the transition can feel frustrating. A body pillow or a “three-quarter prone” position, where you’re almost on your stomach but slightly angled onto one side with a pillow under your hip, can replicate the feeling without putting direct pressure on your belly.
Exercise Positions to Modify or Skip
Exercise during pregnancy is safe and beneficial for most women. The American College of Obstetricians and Gynecologists encourages pregnant women to continue or start physical activity unless they have specific medical complications. That said, certain positions and movements need modification as pregnancy progresses.
After the first trimester, exercises performed flat on your back (like crunches, bench presses, or certain Pilates moves) carry the same vena cava compression risk as back sleeping. Switching to an inclined position or performing exercises on your side eliminates this concern. Intense abdominal exercises that create high pressure in your core, like full sit-ups, double leg lifts, or heavy overhead lifting, can strain the connective tissue running down the center of your abdomen and may worsen a natural separation of the abdominal muscles that occurs during pregnancy.
Yoga is generally safe, and many poses once thought to be off-limits, including downward-facing dog, child’s pose, and happy baby, have been studied without negative effects on the mother or baby. Balancing poses like half moon, warrior III, and tree pose may need modification with a wall or chair, since your center of gravity shifts significantly as your belly grows. Research shows that about 10% of pregnant women from 16 weeks onward already have difficulty with movements requiring forward and overhead reaching.
A hormone called relaxin, which peaks during the first trimester, loosens your muscles, tendons, and ligaments to prepare your body for birth. This increased laxity makes it easier to overstretch a joint without realizing it, so deep stretches and extreme flexibility work should be approached with caution throughout pregnancy. Hot yoga, where the room can reach 105°F, is not recommended at any point during pregnancy due to overheating risks.
Standing and Sitting Posture
Prolonged standing is a real concern during pregnancy, particularly in the second and third trimesters. As your blood volume increases and the growing uterus puts pressure on pelvic veins, standing in one place for extended periods causes blood to pool in your legs and forces fluid into the tissues of your feet and ankles. Symptoms of varicose veins and leg swelling tend to worsen with long periods of standing and with each successive pregnancy. If your job or daily routine involves a lot of standing, taking short breaks to sit, walk around, or elevate your feet can provide significant relief.
Sitting posture matters too, especially if you develop pelvic girdle pain, which affects a significant number of pregnant women. Both slouching and sitting rigidly upright can strain your back and pelvis. The goal is a relaxed, supported posture: sit back in your chair rather than perching on the edge, use a small cushion behind your lower back, and keep your feet flat on the floor. Crossing your legs can aggravate pelvic discomfort by creating uneven pressure across the pelvis.
Getting Up From Lying Down
How you transition between positions matters almost as much as the positions themselves. Your shifting center of gravity makes sudden movements riskier for falls, and sitting straight up from a flat position puts unnecessary strain on your abdominal muscles. Instead, roll onto your side first, then use your arms to push yourself up to a sitting position. This technique reduces pressure on the midline of your abdomen and also avoids the brief moment of back-lying compression that comes with trying to sit straight up from the floor or bed.
This side-roll technique is especially useful in the weeks after delivery as well, when the abdominal muscles are still recovering and need time to close any separation that occurred during pregnancy.