The odds against giving birth to twins are 80 to 1, against triplets 6,400 to 1, against quadruplets 512,000 to 1 and against quintuplets 40,960,000 to 1. Last week in Mexico City, Mrs. Maria Teresa López de Sepulveda, 21, wife of a social security clerk and mother of a two-year-old son, produced the 20,971,520,000,000-to-l chance: octuplets, four boys and four girls. The babies, weighing about 19½ oz. each, were two months premature, and all died within hours. It was only the third time in this century that octuplets have been reported (the others: 1921 in Mexico, 1934 in China) and the first time such a birth has been confirmed. As for those fertility pills doctors suspect of contributing to the recent rise in multiple births, Mrs. Sepulveda had never even heard of them. But she had been taking birth-control pills until eight months before conceiving her octuplets.