How common tearing is, and what the degrees mean
Most first-time mothers have some tearing.
Tears are graded by depth. A first-degree tear involves skin only. Second-degree involves the muscle underneath and is the most common tear needing stitches. Third-degree involves the anal sphincter muscle. Fourth-degree extends through the sphincter into the bowel lining. Third and fourth together are called obstetric anal sphincter injury, or OASIS.
In 7,096,056 US vaginal deliveries, third-degree tears occurred in 3.3% and fourth-degree in 1.1%, a combined rate of about 4.4%. Rates are much higher in first births and after forceps.
In England, the rate of severe tearing in first births rose from 1.8% in 2000 to 5.9% in 2012. The authors attributed most of that rise to clinicians recognising and classifying injuries properly, not to more injuries occurring.