Yes, the more your weight is outside your optimum range, the more it will adversely affect your chances. This goes for both underweight and overweight. Quick weight gain and loss can also affect ovulation and your hormone levels. Body Mass Index (BMI) is a measure of weight in relation to the height measured in Kg/height in meters squared. BMI outside the normal range of 18.5-22.5 has been shown to affect the chance of pregnancy.