Introduction The absence of tolerance in the levels of carbohydrates at the onset or at the time of pregnancy amongst females is known as gestational diabetes mellitus (GDM). This study is designed to determine the frequency of GDM and factors responsible for GDM to assess the actual magnitude of the outcome. Furthermore, it allows for developing strategies to minimize morbidities and improve the pregnancy outcome by early diagnosis and timely management, which can help reduce the frequency of GDM. The aim of the study was to determine the frequency of GDM and the factors responsible for GDM. Methods This was a cross-sectional study conducted in the Department of Obstetrics & Gynaecology Unit 2, Civil Hospital Karachi from the period starting from March 1, 2017, and ending on August 31, 2017, in order to determine the prevalence and associated risk factors of GDM. The study was carried out on 674 pregnant women. A total of 185 consecutive booked cases between the ages of 20 and 40 years, with parity 1 or more with gestational age greater than 24 weeks, were included in the study. Fasting plasma glucose levels 5.1-6.9 mmol/L (92-125mg/dl) and two hours plasma glucose levels of 8.5-11.0 mmol/L (153-199mg/dl) were set up as cut-off levels. GDM and factors were labeled on the basis of cut-off levels. Factors responsible for GDM included high maternal age when the age of the women was greater than 35 years and grand multiparity when women having a number of children greater than five, that is, women who have given birth five or more times. The collection forms were completed in the postpartum period. All information was obtained through the patient's clinical record and prenatal card. Initially, all variables were analyzed descriptively. To see the association of the groups, the chi-squared test (χ2 test) or Fisher's exact test was used. The level of significance used for the tests was 5%. Results The prevalence of gestational diabetes was 9.73% (95% CI: 8.53-12.64). The average age of the patients was 28.99 ± 4.34 years. The average pre-gestational BMI was 25.44 ± 2.74. Out of 185 women, 127 (68.65%) were less than or equal to 30 years of age. The average pre-gestational BMI was 25.44 ± 2.74, and average gestational age was 28.99 ± 2.34 years, respectively. A total of 161 (87.03%) of the women had a family monthly income of more than 10,000 PKR. There were 61 (32.97%) primiparous, 97 (52.43%) multiparous, and 27 (14.59%) grand multiparous women. Most of the women were illiterate numbering 36 (19.46%) or primary educated, numbering 30 (16.22%), and secondary educated or higher numbering 6 (3.24%). High maternal age (>30 years), high parity (>3), previous history of GDM, and family history of GDM were the significant factors of GDM. Conclusion The results of our study showed that the prevalence of gestational diabetes was 9.73% (95% CI: 8.53-12.64). Therefore, this study also showed that developing GDM was directly related to the followi...