This study estimates the number of 1st births that occurred before the 8th month of marriage in Switzerland. Available official statistics show that the majority of births before the 8th month of marriage, or the number of premarital conceptions, have caused the marriage to happen, and were not conceptions planned in view of an impending marriage. A method of calculation to estimate the number of premarital conceptions is proposed; the method estimates 1st births according to age of mother and length of marriage, and then uses the results from these estimations and the known number of births between the 8th and the 11th month of marriage to estimate the number of premarital conceptions that are related to the age of the mother. Results show that the proportion of marriages with premarital conceptions is highest at age 19. After 1968, however, the highest percentages are around 19 and around 29. The proportion of provoked marriages around 19-20 increased regularly between 1945-1960, to reach a maximum level around 1961-65; the proportion decreased between 1967-72 and increased slightly after 1973, especially in women after 30. According to the authors these results show a more effective use of contraception after 1968. It is possible that the highest level of premarital conceptions around 19 corresponds to "accidents" which provoke the marriage, while the other highest level corresponds to wanted conceptions for couples who wish to legalize their relationship before the birth of the child.