Although China's population growth rate in 1980 was the lowest in 31 years, doubling of marriages and increasing life expectancy have emerged as threats to the government plan to hold national population growth to 1200 million by the end of the present century, a Beijing official magazine reported.  Liaowang (Observation), published by the government's Xinhua news agency, quoted state family planning officials as saying that a subsequent doubling of marriages this year "threatens another baby boom in the world's most populous nation."  The officials said the country's population stood at 982.55 million at the end of 1980, roughly equivalent to 20% of the world's total.  According to the magazine, the natural population growth rate declined to 8.97 million last year as the number of births fell to 14.99 million and that of deaths stood at 6.02 million.  During the past decade, the natural increase rate of China's population dropped from 26/1000 in 1970 to just 12/1000 in 1980.  Owing to increase in production and living standards, along with expansion and improvement of medical facilities, the mortality rate fell from almost 30/1000 before the founding of the People's Republic of China to around 6-8/1000 by the 1970s.  According to U.N. statistics prepared for the recently-held international conference on population and development in Beijing, China succeeded in reducing its birth rate by as much as 23.1% between 1960-65 and 1970-75.  But such accomplishment is now in danger of being wiped out by a new birth peak expected during the next 10 or more years as those born during the baby boom of the 1960s reached childbearing age during the present decade.  In addition, the population control program is also threatened by the rising life expectancy of the Chinese people.  Latest statistics compiled by the Ministry of Public Health showed that the average life span of the Chinese population had doubled to 68 years in 1980 from 35 years in 1949.