Laurie Glenn Jacobson, 5 1/2 months with child, slung her M-16 rifle over her shoulder at Quantico, Va., Marine Corps Base and lowered herself down the rope net, one of the first pregnant women to undergo rigors of Marine training.

Jacobson, a 5-foot, 118-pound second lieutenant, is nearing the end of a 21-week officer training course legendary for its grueling obstacle courses, long marches and mock battles and, until a year ago males only.

Though it is still something new for the two-fisted Leatherneck to be sharing rank and ration with expectant mothers, Marine officials said that of 5,000 pregnant women in the Corps today, 44 are pregnant and have chosen to say on active duty.

A spokesman also said that with 10,000 women expected in the Marines' ranks by 1985, headquarters is at work on a Marine maternity uniform.

That will come too late for Jacobson, whose fatigues, the only military apparel she can still fit into, are size extra small short - the same size her husband, a Marine first lieutenant now stationed on Okinawa, wears.

"The baby wasn't any problem - I'm just so short I had to take twice as many steps just to keep up," said Jacobson, who two weeks ago marched 20 miles across the rugged terrain of the Marine Corps Development and Education Command at Quantico with helmet, seven-pound rifle and 25-pound pack.

"I felt so good afterwards. It was worth it," she said.

Jacobson's combat boots had to be specially made to a man's size 3.

"But she never slowed up a march," said 2nd Lt. Mark Howard, who added that he still believes the Marines are for men only.

Jacobson is one of 15 women in the 244-person Charlie Company, and the only women in the company who is pregnant.

Until July 1975 woman Mariners who became pregnant were automatically discharged from the service. Now official Marine policy is to leave the matter up to the woman and her examining physician, and to intervene only when the health of the mother or child is endangered or when the mother can no longer physically perform her duties.

"I believe my body will let me know when I'm doing something I shouldn't do," said Jacobson. The only exercise Jacobson did not fully participate in was the "Three-Day War" - a simulated battle exercise in which combatants are exposed to tear gas.

"The doctor told me not to take aspirin, so I thought tear gas would be a bad idea," said Jacobson.

Jacobson, 25, said she left her job as a technician and part-time director at a Wilmington, N.C., television station to become a Leatherneck because of the promise of equal pay and equal opportunity.

She enlisted in January, and upon completion of officer candidate school, was commissioned. She is now one week away from completing the course known as TBS, The Basic School.

The problem women face in the Marine Corps are no different from those faced by women in the civilian world, Jacobson said. What she did, she was quick to assert, was done "because I'm a Marine and it's what I'm supposed to do," not because of any intent to make a political statement.

When the men in Jacobson's platoon learned she was pregnant, "they were sort of patronizing. They asked if they could lift things for me. Now they don't bother asking," said Jacobson who last week shouldered a machine gun tripod and 400 rounds of ammunition during a field exercise.

"But tradition dies slow and change comes hard in the Marine Corps," noted one sergeant on the base, who admitted that he would prefer the Marines stay all-male. "It's a man's Marine Corps. It is amacho and it always will be," he said.

Women Marines are forbidden by law from taking an active combat role, but their training and lifestyle at Quantico are indistinguishable from those of the men.

Jacobson lives in the same barracks as the men, but has a single room, as do many for men. She has been shown no special treatment, her superior officers insisted.

For Jacobson the toughest moment was not in the field but a formal dinner in the mess hall when, unable to fit into formal white uniform, she entered wearing a long, coral colored evening gown.

"I felt very out of place and wrong. A couple of generals came up and asked me what my problem was. Once I told them I was pregnant they said, 'Great, congratulations,'" she said.