Kenya has achieved the dubious distinction of becoming the first country to record a 4 percent annual rate of natural increase, a figure that means its 16 million population will double in 18 years.

Population specialists say the rapid growth is Kenya's most serious development problem and they cite "staggering, mind-boggling" difficulties ahead for one of Africa's more advanced and better-governed countries.

Though 4 percent may not sound like much, it means, at an individual level, that the average Kenyan woman has 8.1 children. Surveys show that no matter how many children a typical woman here has, she wants more.

The populations growth rate could wipe out this East African country's plans for long-term economic advancement because it means the economy has to expand by 4 percent a year just to stay even. It grew by only 2 percent last year.

In the last two years Kenya, known for its agriculture, has been unable to feed itself and this year will import more than half a million tons of grain, partly because of drought. In 1981 there will be almost 650,000 more mouths to feed than in 1980. Even the most optimistic of forecasts for agricultural advancement project that Kenya will have to import food for most of the decade.

Many African countries face similar difficulties in the future because of soaring birthrates twinned with declining agricultural output.

"Kenya has serious problems right now," said Spencer Silberstein, a populaton expert for the U.S. Agency for International Development. Pressures on land in some areas of Kenya, he said, are approaching that of Bangladesh or Indonesia, whose birthrates are now declining. Kenya's rate is increasing.

Zimbabwe is second to Kenya in black Africa, with a growth rate of about 3.4 percent -- meaning the population will double in just over 20 years.

The difference is that Zimbabwe still has millions of acres of underdeveloped arable land and is rich in mineral resources. Kenya is slightly smaller than Texas, but less than 20 percent of its land is arable and 75 percent of the population lives on 10 percent of the land. Most of it has already been exploited and there are no known major deposits of minerals.

In some respects, Kenya, one of black Africa's most advanced countries, is a victim of its own progress. It maintains better statistics than most countries on the continent, so others could be growing as fast without knowing it.

More important, population is increasing at a faster rate than in other African nations because the death rate is lower. Life expectancy is 56 years, the highest in mainland black Africa. The country's death rate of 14 per 1,000 and infant mortality rate of 83 per 1,000 are the lowest, according to the Washington-based Population Reference Bureau.

However, there are 53 births for every 1,000 people, the highest rate in the world. By contrast, the U.S. figure is 16.

Silberstein noted that Kenya is one of the few African countries to have embarked on a family planning program. "Services have greatly expanded, but the results have not," he said, adding that "there is still no vigorous government program."

Only about 3 percent of Kenyan women practice any modern method of birt control and most of those use it to space, not limit, children.

Salvator Kanani, the Ministry of Health official in charge of family planning, said the program was first announced in 1967. In 1972 a study was carried out. In 1975 a mother-child health program was started. Last year another study was completed but it has not yet been presented to the Cabinet.

There is still no separate budget for family planning; so far, efforts are almost entirely informational with little distribution of birth control devices.

Since the program was announced in 1967, the population has shot up from about 10 million to 16 million and the growth rate has soared from about 3.5 percent to a World Bank estimate of 4 percent.

Kanani disputed the 4 percent figure, saying he thought it was about 3.5 percent, but he provided no evidence.

President Daniel arap Moi, who has seven children, frequently tells people at rallies that they should limit their offspring to the number they can support. Charles Njonjo, a key Cabinet minister and supporter of Moi, told a meeting last year that Kenyans should stop "breeding like rabbits."

Kanani, a bachelor, said, "Things are moving fast enough" and it "is unrealistic to expect dramatic changes in five years."

Silberstein said it would take a "tremendous effort over several decades" to reduce the fertility rate of 8.1 to, say, the 5.3 rate that India has achieved, "which is still grossly inadequate."

African officials often point out that the population problem must be examined in a wider context.

"Dishing out contraceptive devices is not the answer on its own," said Simon Mazorodze, Zimbabwe deputy minister of health, in a recent interview.

"People want more children for insurance for their old age, because of the high infant death rate" and to provide workers on their farms, Mazorodze said. "Public education and increasing the standard of living is the answer," he said.

A Western diplomat in Nairobi disagreed with this approach.

"You can't have family planning without birth control devices," he said. "Availability, availability," is the key, he said. "Get it out there."

"The backdrop for every problem in this country is population," he added. "It just keeps ticking along."

The problem is that, in many parts of Africa, numbers translate into political power. In Kenya, where tribal politics has always been important, the number of Kikuyu, the largest tribe, increased by almost 50 percent between the 1969 and 1979 censuses, while the population of Luos, their rivals, only rose by about 25 percent.

Many Kenyan elite have large families contrary to the usual trend for the wealthy to have fewer children.

Some are having second thoughts, however. James Mwanthi, a 46-year-old businessman, was at first reluctant to say how many children he has. He has eight ranging from 2 to 19 years old.

"No more children," he said. "That's the end of the road."

"Women in Kenyan society don't become a person until they have children," Silberstein said. "Their value is as a mother so if they don't produce significant numbers of children they are failing."

Few fail. As a result, the sluggish economy must create 260,000 new jobs a year. By the year 2000, if present trends continue, there will be a need for 560,000 new jobs a year. In a country where 50 percent of the population if under 15 (it is 22 percent in the United States), the number of school children will more than double to 7.1 milllion by the end of the century.

Arable land per capita in the high and medium agricultural areas has almost halved since independence in 1963 to about 1.5 acres per person. At current rates of growth it will be down to two-thirds of an acre, hardly viable for a farm, by the year 2000.

"If the present population trend continues, the portents are for disaster," Silberstein said. "If all other African countries go through the stages Kenya has [in family planning], the whole continent is doomed."