A separation of the right and the left sides of the rectus abdominis muscles is a condition known as diastasis recti. According to DeGowin’s Diagnostic Examination, physicians often uncover the condition when a patient raises his head and tenses his abdominal muscles. A longitudinal ridge or bulge is evident in the contracted abdominal muscles. Diastasis recti can be congenital or acquired.

Diastasis recti is a common condition in newborns, especially in premature and African American babies. The condition occurs when the rectus abdominis muscles insert in the ribs farther from the midline than normal. According to Rudolph's Pediatrics, caregivers may often see a separation between the rectus muscles in the midline when the infant is crying.

Diastasis recti is more commonly acquired. Often the abdominal wall cannot withstand the pressure that occurs in conditions such as multiple births, high-birth-weight infants, repeated pregnancies, poor abdominal tone, obesity and aging.

Excess pressure on the abdominal walls in pregnant women with multiple fetuses or in women who have repeated pregnancies can cause diastasis recti and complicate the delivery, according to Williams Obstetrics. In these cases, fat, skin, peritoneum—a thin flap of tissue that covers the internal organs—and a stronger tissue called fascia form the abdominal wall. No muscle layer is present.

According to the Merck Manual for Healthcare Professionals, diastasis recti can also occur in trisomy 18, a genetic disorder in which hernias, cryptorchidism and increased skin folds around the neck also are common. Cryptorchidism is a condition in which one or both testicles do not to descend into the scrotum. An abnormally clenched fist, clubfeet or rocker-bottom feet and severe congenital heart disease and anomalies of the lung, ureters, diaphragm and gastrointestinal tract are often present in trisomy 18. Trisomy 18 is also known as Edwards' syndrome or trisomy E.