Abstract: The first visible outcome of the fertility process is the birth of the first child. The first birth marks a woman's transition into motherhood. It plays a significant role in the future life of each individual woman and has a direct relationship with fertility. The age at which child bearing begins influences the number of children a woman bears throughout her reproductive period in the absence of any active fertility control. For countries in sub-Saharan Africa, where contraceptive use is relatively low, younger ages at first birth tend to boost the number of children a woman will have. However, even when family planning is widespread, the timing of first births can affect completed family size if contraception is used for spacing but not for limiting fertility. The birth of a child is an event of great social and individual significance and its importance is recognised in all human societies. It signifies the transition of a couple into a new social status, i.e. parenthood with its related expectations and responsibilities. It marks the sexual and social maturity of the mother and the visible consummation of sexual intercourse . The relationship between age at first birth and overall fertility in developing countries is generally an underdeveloped area as far as demographic research is concerned. Fertility analysts generally assume that child bearing only occurs within marriage. Then they treat age at first marriage to be a major proximate determinant of fertility. This assumption might have been true in most traditional societies, where births out of wedlock were not accepted and virginity was a prerequisite for marriage. This assumption, however, does not hold true in modern times, where a large number of children is born outside marriage. These facts have been the major motive of conducting the current research. This study examines the reproductive behaviour of Tanzanian women. The study found the average age at first sexual intercourse to be 16 years; age at first marriage to be 17 years, while average age at first child bearing was estimated to be 18 years. By age 15, almost 10 percent of juvenile women have given birth. This study furthermore found that 41 percent of all first live births resulted from premarital conceptions. Out-of-wedlock births account for 24 percent of all first births in Tanzania according to the 1996 TDHS. The education of a woman, place of residence, and religion play the greatest roles in influencing age at first birth in Tanzania. The striking results were place of residence as it was found that rural residents have a higher mean age at first birth than women living in Dar es Salaam. Dar es Salaam women has the lowest mean age at first birth. Moslems have lower age at first birth than Catholics. There is also a strong relationship between age at first birth and age at first intercourse also with infant and child mortality. The results indicate that the younger the age of the mother at the birth of the first child, the higher the chances that the child dies. The study of current levels and trends of fertility showed that, on the average, a Tanzanian woman bears 6 children. Between the 1960s and early 1980s, an average of 7 births per woman prevailed in Tanzania. The declining fertility in Tanzania was confirmed by the analysis of the decomposition of the change of the total fertility rate (TFR) between two TDHSs. It found that natural fertility control is being gradually replaced by contraceptive use although the percentage of women using contraception is still very low. The reduction in infant and child mortality, rising numbers of women attending schools, and a rise in age at first birth are among the factors responsible for the decline in fertility in Tanzania. Education on the other hand has played a major role in raising age at first birth. Education either delays first intercourse and subsequently birth as pregnant schoolgirls are prohibited to attend formal schools or it effects the acceptance of contraception to delay first conception. Specifically, this study establish that there is an inverse relationship between age at first birth and fertility. Marriage is a weak factor in explaining fertility in Tanzania. Women in polygamous unions had fewer number of children ever born than those in monogamous unions in Tanzania. Moslems have a lower fertility than Catholics. Women residing in urban areas have fewer children than those in the rural areas. Women's education is the strongest predictor of the use of contraceptives in Tanzania. Catholic women are less likely to use contraceptives than Moslems. Tanzanian women residing in rural areas are less likely to use contraception than their counterparts residing in urban areas. Although age at first birth did not show any significance, age at first intercourse, age at first marriage and current age are highly related to contraceptive use. Variation in age at first sexual intercourse; marriage; and birth, and the extent of practising contraception are found to depend mainly on religion, place of residence, and the age of a woman. It is important therefore to design separate programmes to raise age at first birth and to lower fertility according to the findings related to religion, place of residence, and target juvenile women separately.