A hospital-based study was undertaken in Harare, Zimbabwe to estimate the proportion of unplanned or unintended pregnancy among mothers who delivered at the referral hospital and to analyse their socio-demographic pattern and sexual relationships. A case-referent study design was used with systematic sampling of maternity records of mothers who had delivered. Interviews were performed before discharge using a semi-structured questionnaire. Mothers who reported that the index pregnancy was unplanned or unintended constituted the cases, and the referents were those reporting the pregnancy as planned. Out of 923 deliveries, 41% were unplanned and 9% unwanted. The mean age of the mothers was 25 years and women aged 19 or below [(Odds Ratio OR) = 2.2, 95% CI = 1.5–3.2] and 35 or above (OR = 2.8, 95% CI = 1.7–4.6) were significantly more likely to present with unplanned pregnancy. Nulliparous women (OR = 2.4) and mothers with five or more pregnancies (OR = 8.2) had a significantly increased likelihood of the pregnancy being unplanned. Level of education in the mothers studied had no independent association to planning of pregnancy. Unemployed (OR = 14) and single (OR = 7.8), or divorced/separated/widowed (OR = 5.1) women as well as those with low income (OR = 2.1) and whose partner earned no income (OR = 2.2), were more at risk of unplanned pregnancy. Those living with their own parents despite being pregnant were also more likely to report an unplanned pregnancy. In conclusion, there are documentable social and reproductive factors underlying unwanted pregnancy. Risk factors for unplanned pregnancy form a pattern similar to those for maternal mortality. Thus unplanned pregnancy is a major indicator of the presence of factors known to increase the risk of maternal death. Policy makers and health education should address factors contributing to unplanned pregnancy and its prevention in order to prevent reproductive mortality and morbidity. The presence of those factors associated with unplanned pregnancy at booking or delivery should also alert service providers to the need for appropriate contraceptive counselling as part of post delivery care.