The prevalence of obesity in Western societies is on the rise, which has a profound effect on the reproductive performance of women who are trying to have a baby. Recent evidence suggests that obesity in women of reproductive age is associated with decreased birth rates, increased miscarriage rates, lowered IVF success rates, higher rates of premature delivery and a marked increase in pregnancy complications. We use two parameters to measure height-weight relationship. The first is Body Mass Index (BMI) (the ratio of height to weight that is calculated by taking body weight in kg and dividing this by the square of height in meters. The second is the percentage contribution made by fat to overall body mass BMI: A BMI of <20 is regarded as underweight; 20 - 25 is ideal/normal.  A BMI of >30 is definitively indicative of being overweight. A BMI of 30-40 is obese and > 40 is morbidly (dangerously) obese. The “ideal BMI” for fertility is 20-25. A BMI of<20 increases the risk of miscarriage. Women with a BMI of > 30 fertility often have a reduction in response to ovarian stimulation. BMI significantly impacts male fertility as well. Men with an increased BMI often experience significant sperm dysfunction. Percentage Fat: The contribution of fat to body weight is also important. The normal contribution of fat to body mass in adult women is about 28%. Anything under 22% can result in dysfunctional or absent ovulation. Both diet and exercise modification can help regulate the percentage of body fat and BMI.Simply stated, being overweight with a significantly elevated BMI and/or having a high percentage of body fat has a decidedly adverse effect on overall reproductive performance and fertility. When it comes to IVF in specific, while the results of several studies have been discordant, the general trend strongly suggests that women who are moderately overweight (BMI>25-30) and those who are obese (BMI>30) have worse IVF outcomes than the controls with a BMI <25. As alluded to above, it would appear that women with a BMI of >25 and especially those with a BMI of >30 exhibit a poorer ovarian response to fertility drugs (impaired follicle and embryo development with fewer blastocysts becoming available for transfer). These women also tend to have a reduced ability to implant transferred embryos into their uterine linings, perhaps due to reduced endometrial receptivity.  Women with polycystic ovary syndrome (PCOS) often usually have BMI’s of >30.  In such women, the hormonal environment in the ovaries is known to adversely affect follicle and egg development, hindering fertility. Given that there is often no clear cut distinction between PCOS and other overweight women, it is possible that many of the factors that are believed to affect egg/embryo quality in PCOS might similarly affect egg development and endometrial receptivity in overweight women. Such factors could include increased production of luteinizing hormone (LH), hyperinsulinemia and increased production of ovarian male hormones (androgens such as testosterone.)  The link between increased LH and resulting increased production of ovarian androgens (mainly testosterone) and poor follicle and egg development is well established.  It is also well known that such hormonal changes can be transmitted to the adjacent uterus thereby adversely affecting endometrial development.  Clearly the question arises as to whether the negative effect of an elevated BMI (>25) on general fertility potential and IVF outcome is due to compromised egg development, endometrial receptivity to the implanting embryo or both.  In my opinion, while a direct ovarian influence probably predominates, there is also likely to be an adverse influence on endometrial development.  This endometrial affect is commonly seen in PCOS women who, when they develop severe ovarian hyperstimulation on fertility drugs often have a very thin (< 8mm endometrium).There is also the reality that it is often technically more difficult to perform a “smooth” and “flawless” embryo transfer in women who are overweight. This fact is especially true when it comes to those with a BMI of >30. Visualizing the cervix is much more difficult and the introduction of the embryo transfer catheter through the cervix, often difficult. Given that the efficiency by which ET is conducted, represents a rate-limiting determinant of IVF outcome, it follows that obese women tend to have poorer overall IVF outcomes. Finally, it is important to emphasize that overweight women are at far greater risk during pregnancy than are women of normal body weight. As previously mentioned, the miscarriage rate is much higher, in addition to an incidence of diabetes, high blood pressure, preeclampsia, premature labor, surgically assisted deliveries, stillbirth and neonatal death. Maternal complications that occur after birth of the baby (i.e., infection, uterine post partum hemorrhage, etc. are also much more common.  Babies born to such mothers, are also at great risk of developing respiratory distress syndrome (RDS). This condition, which ordinarily only occurs in preterm babies, can also occur in the absence of prematurity in such cases. RDS is the most common reason for the newborn having to be admitted to a neonatal intensive care unit, and also the most common cause of death in the first week of life.  The clinical significance of a growing population of overweight women is enormous because not only can this compromise their overall reproductive performance but it also compounds the risk of chronic medical conditions such as diabetes, coronary/cerebral/peripheral vascular disease and thus compromises life expectancy as well as the quality of life.  As such, being overweight represents an overall life hazard that should be addressed by the medical profession as well as by society as a whole.  The answer is surely not a simple one but the solution does not lie in dieting alone (which rarely is of sustained benefit). Instead it requires an overall modification in lifestyle.