The United States surgeon general on Thursday issued a public warning that smoking or vaping marijuana is dangerous for pregnant women and their developing babies.

At a news conference with other top Trump administration health officials, the surgeon general, Dr. Jerome Adams, said he was concerned that pregnant women, teenagers and others were unaware of the health hazards posed by new, professionally grown marijuana crops.

Retail outlets have recommended marijuana use for morning sickness, the surgeon general’s advisory noted. And doctors are concerned that T.H.C., the high-inducing chemical in marijuana, poses risks for the developing fetal brain, and that it could be passed to infants through breast milk.

Dr. Adams described modern marijuana as far more potent than marijuana produced and sold 20 years ago, with levels of T.H.C. increasing to a range of 12 percent to 25 percent from 4 percent back then.