Placing a pillow between your legs during pregnancy keeps your hips level with your pelvis, which prevents your spine from twisting and takes pressure off your lower back, hips, and pelvic joints. This simple adjustment addresses several overlapping problems that get worse as pregnancy progresses: hormonal changes loosen your joints, your growing belly shifts your center of gravity, and side sleeping (the recommended position) can pull your upper leg downward and strain everything from your lower back to your knees.
What Happens to Your Joints During Pregnancy
Your body produces a hormone called relaxin throughout pregnancy, and its job is exactly what the name suggests: it loosens your ligaments to prepare your pelvis for delivery. But relaxin doesn’t target only the pelvis. It affects connective tissue throughout your body, creating increased joint laxity and, in some cases, genuine joint instability. The ligaments that normally hold your pelvis, hips, and spine in place become softer and more flexible than usual.
This is useful for childbirth but creates real problems for sleep. Between 50 and 80 percent of pregnant women experience low back pain, and it frequently disrupts sleep. When you lie on your side without support between your legs, your top knee drops toward the mattress. That downward pull rotates your pelvis, tugs on already-loosened ligaments, and compresses the hip you’re lying on. Over several hours, this can cause or worsen pain in your lower back, hips, and the joint at the front of your pelvis (a condition called pelvic girdle pain or symphysis pubis dysfunction).
How a Pillow Restores Spinal Alignment
The goal is to keep your upper leg parallel to your lower leg so both are level with your pelvis. When a pillow fills the gap between your knees, it prevents your top leg from dropping and rotating your spine. Your pelvis stays in a neutral position, your lower back isn’t twisted, and the muscles along your spine can actually relax instead of working to stabilize a misaligned joint all night.
For the best effect, the pillow should support more than just your knees. Placing it between your thighs, knees, and feet (or using a long pillow that spans the full length of your legs) keeps your entire lower body aligned. If only your knees are cushioned, your ankles may still collapse inward, which can reintroduce some of that rotational strain. Think of it as making your top leg a mirror image of your bottom leg.
Circulation and Blood Flow
Side sleeping itself is important during pregnancy, especially in the second and third trimesters. When you lie on your back, the weight of your uterus compresses a major vein called the inferior vena cava, which carries blood from your lower body back to your heart. This compression reduces the amount of blood returning to your heart, lowers cardiac output, and can impair blood flow to the uterus. Since the 1950s, the left lateral (side-lying) position has been standard practice for displacing the uterus off this vein and improving circulation.
A pillow between your legs makes side sleeping more comfortable, which means you’re more likely to stay in that position. If side sleeping feels painful or awkward, the natural tendency is to roll onto your back during the night. The pillow removes one of the biggest barriers to staying on your side: hip and back discomfort.
Relief for Pelvic Girdle Pain
Pelvic girdle pain affects a significant number of pregnant women and typically involves aching or sharp pain at the front of the pelvis, the lower back, or the hips. It tends to worsen with activities that involve uneven weight distribution, like climbing stairs, standing on one leg, or rolling over in bed. Sleep is a common trigger because of how long you spend in one position.
Ireland’s Health Service Executive specifically recommends lying on your side with a pillow between your knees to manage this pain. As pregnancy progresses, adding a second pillow or rolled towel under your belly can further reduce strain on your hips and lower back by preventing your abdomen from pulling you forward and down.
How to Position the Pillow
A standard bed pillow works, though it may shift during the night. Fold it in half for more thickness if needed. The pillow should be thick enough that your top knee sits at the same height as your hip, not higher or lower. If you find that a regular pillow bunches up or slides out, a longer body pillow gives you continuous support from your chest down to your ankles.
Dedicated pregnancy pillows come in several shapes. U-shaped pillows cradle your entire body, supporting your head, back, belly, and hips simultaneously. They’re helpful if you tend to roll between sides during the night because the support is on both sides. C-shaped pillows are more compact and work well for targeted support. They tuck between your knees to relieve hip and lower back pain while also curving up to support your belly. The right choice depends mostly on your bed size and whether you want full-body support or something less bulky.
A Simple Setup
- Between your legs: A firm pillow spanning from mid-thigh to ankles, thick enough to keep your top leg level with your hip.
- Under your belly: A small pillow or rolled towel to prevent your abdomen from pulling downward, especially in the third trimester.
- Behind your back: Optional, but a pillow wedged behind you can prevent you from rolling onto your back during sleep.
When It Helps Most
Most women start noticing the benefits in the second trimester, when the belly becomes heavy enough to shift their center of gravity and relaxin levels are high enough to affect joint stability. By the third trimester, the combination of added weight, loose ligaments, and the need to avoid back sleeping makes leg support feel close to essential for many women. That said, there’s no reason not to start earlier if side sleeping already feels uncomfortable. The technique works the same way whether you’re 14 weeks or 38 weeks along.