Property and casualty insurers’ premiums grew by a small amount last year, but earnings were hammered by an unprecedented number of natural catastrophes, weak investment returns and a generally sour economy, A.M. Best Co. Inc. said in a report released yesterday.

Commercial property/casualty underwriters experienced an estimated $15.2 billion in underwriting losses last year, the New Jersey-based rating agency said.

Though greater industry focus on underwriting and pricing discipline boosted net premiums written by an estimated 3.9 percent to $181.2 billion in 2011 — the segment’s first year-to-year increase since 2004 — net income for commercial property/casualty insurers dropped an estimated 40.3 percent to $11.7 billion in 2011, according to the report.