Clients who are pregnant should NOT be in a supine position for more than 5 minutes.
The major blood vessels supplying the heart run along the right front side of the spine. As pregnancy progresses, the weight of the fetus begins to compress these vessels when the client lies on her back. Compression of the vessels can interfere with return blood flow to the heart, resulting in nausea or fainting.
In addition, clients who hold their breath for too long (up to 8 seconds with a maximal exertion), can pass out. When clients hold their breath on exertion, the internal pressure in the chest and abdomen increases. A significant increase can constrict the blood vessels that supply the heart, causing fainting.
BMC Pregnancy Childbirth. 2018 Nov 12;18(1):440.
