We analyzed the data on 55,839 births that taken place in 1998 year in the southeast region of Poland. The incidence of twin pregnancy was about 1%. A half of them delivered before 259 day of gestational age, the beginning of birth occurring norm for human. This means that onset of this norm for singleton pregnancies is the mean of 12 weeks births occurring norm (215-303 days of gestational age) for multifetal pregnancies simultaneously. The particular meaning has prevention of preterm delivery and in this case the computer-aided monitoring of pregnancy and prediction of birth date is a very useful invention.