Introduction: Unwanted pregnancy is one of the most important public health problem worldwide and causes much stress and unhappiness for the woman and her partner and possibly for the child if the pregnancy continues. As a result it could be a problem for society as a whole. According to the statistics, worldwide 75 million out of 175 million pregnancies are unwanted and 45 millions end up with abortion each year. This study aims to way up the rate of unmet needs (needs for contraception but not using them for a reason) and the causes of them. It also aims to answer to this question, that why women are not able to chronolgically plan their pregnancies and nursing their children. Materials and Methods: This is a cross sectional study that aims to investigate the extent and the causes of unplanned pregnancies, the use of contraceptive methods and the reasons for not using them in order to explore the effective factors of unwanted pregnancies and measuring the rate of unmet needs that all affect on the control of population growth rate in Tehran, Iran. A random sample of 168 pregnant women who attended to the women clinics in Tehran, were interviewed in a period of six months (December 1998 to May 1999), using a questionnaire. The questionnaires were completed by the participants. The data were analysed by Fisher Exact Test, 2, lojestic Regression using SPSS software program and the significance level was based at P < 0.05. Results: According to the results, the response rate was 89.28%. Among all of the sampled women 94 (63%) used contraceptions. Out of 150 respondents, 47 (31/3%) had unplanned and 103 (68.7%) had planned pregnancies. Twenty three (49%) of the unplanned pregnant women used contraception and the current pregnancy stemmed from the failure of the contraceptive method they were using. Twenty four (51%) women with unplanned pregnancies said that they did not plan to become pregnant, but for a variety of reasons they were not using contraception. Among demographic variables (women and their partners) the percentage of unwanted pregnancies in women under 20 and above 40 years of age was increased and this relationship showed a tendency to be significant (P=0.056). There was also a significant relationship between the partners education levels and the type of pregnancy (wanted or unwanted, P < 0.05). Moreover, a significant relationship between the number of boys (P < 0.01); girls (P < 0.0001); previous number of pregnancies (P < 0.01) and the current pregnancy being wanted or unwantermd. Conclusion: The low prevalence of contraceptive use in this group indicates the failure of family planning clinics to motivate their target groups in using contraception for preventing unwanted pregnancies. The high prevalence of unwanted pregnancies while using contraceptive methods suggests the need for education to improve women’s knowledge about how and when to use the methods in order to increase their ability to plan their pregnancies.