A dramatic rise in the number of sexual assaults so far this year erupted into a particularly violent past four weeks for women in the District, including a string of serial attacks and the rape and brutal beating this week of a pregnant woman.

Yesterday, as police released composite sketches of two men suspected in seven sexual assaults since Aug. 28, details of the attack on the pregnant woman late Thursday left veteran investigators frustrated and angry.

"You got some sick people out there," said Capt. Wyndell C. Watkins, the commander of the sex offense branch.

In the first six months of this year, police say there have been 128 reported forcible rapes, compared to 77 reported in the same period in 1989. In 1988, police statistics show, the number of reported forcible rapes was 145, nearly as many as were reported in just the first half of this year.

Watkins, referring to the overall rise in rapes this year and not the recent incidents, attributed the increase to the increase in drug use because it forces women addicts to expose themselves to abuse.

Drug use, said Watkins, has left women addicts in a very vulnerable position. They are on the streets late at night, cast "into an arena" where they are more likely to be assaulted sexually, he said.

The latest attack -- which police said is unrelated to the other abductions and assaults -- took place at 11 p.m. Thursday in a secluded area of Piney Branch Park near the 3600 block of 16th Street NW.

Police, responding to a report of screams, found a woman in a grove littered with broken bottles, branches and other debris. Yesterday afternoon, almost 12 hours later, the vicious nature of the assault was evident. A thick branch was covered with blood, as was the ground beneath it, and a bandage left by emergency technicians was stained red.

The woman, who is 30 and about six months pregnant, was beaten so severely on the face and head that doctors at Washington Hospital Center were still not able to assess the extent of her injuries yesterday.

Police searching the area found James Reid, 33, crouching behind some shrubs nearby. He was questioned and initially charged with assault with intent to kill. After tests were performed on the woman yesterday, police also charged Reid, who lives in the 1400 block of Oak Street NW, with rape while armed and mayhem while armed.

The charge of mayhem is unusual, and it is an indication of just how severely the woman was beaten, authorities said yesterday. It is used only when beatings result in some disfigurement or a loss of the "normal functioning of the human body."

The victim, whose right eye appears to have been damaged, is conscious but in critical condition, said Sandra Gregg, a hospital spokeswoman. Her pregnancy remained uninterrupted, but doctors have not been able to conduct a full examination because her condition is not stable enough, said Gregg.

Watkins, who held a brief news conference yesterday afternoon, provided more details on the kidnapping and sexual assaults of 10 women.

One of the attackers has large buck teeth, weighs between 110 and 120 pounds and is about 4 feet 10 inches tall. The other is approximately 25 years old, about 6 feet tall and weighs between 160 and 180 pounds.

The descriptions of the suspects were given by two women who were abducted Thursday at 2 a.m. from Florida and Georgia avenues NW and taken to a playground in upper Northwest, where they were robbed and sexually assaulted.

Watkins said the victims share several similarities: All are black women in their mid-twenties, all were abducted between 11 p.m. and 2 a.m., and all were waiting for transportation or were walking from one location to another.

In at least four of the incidents, the men drove up in a four-door, light blue or gray Hyundai, and offered the women a ride. When refused, they drove around the block, returned and abducted the women at gunpoint, Watkins said.

In two incidents -- Aug. 28 and Sept. 4 -- the men abducted each time three women who knew each other and were standing together, said Watkins. Of these six, five were robbed and one was raped, he said.

The suspects tended to drive the women to the same location. The attacks have occurred twice in the 6200 block of Chillum Place NE and twice behind St. John's College High School at 2607 Military Rd. NW.

Although the 10 women who have been abducted all have been robbed, the primary motive appears to be sexual assault, Watkins said. He cautioned women to be wary and to avoid poorly lit areas late at night.

In response to the attacks, police have increased nighttime patrols. They have not ruled out the possibility that other victims have not come forward, and Watkins yesterday urged women who have been assaulted or similarly approached to call police.

Nationwide, the number of reported forcible rapes increased slightly in 1989, as compared to 1988. The figures, according to the U.S. Justice Department's Uniform Crime Reports, are 94,504 for 1989, and 92,486 for 1988, an increase of 2.2 percent.

In 1989, an estimated 75 of every 100,000 females in the country were reported victims of forcible rape, which by definition applies only to women.