I’ve struggled with weight my entire life. I’m 36.5 years old and I have a body mass index of 37.5. I had two miscarriages in the last year, about six months apart. What effect does my obesity have on my ability to stay pregnant? Does an above average body mass index cause miscarriage?

Written by our medical expert William Buckett, M.D.

Many studies have shown an association between being overweight and a higher chance of miscarriage. Initially this was thought to be only in IVF pregnancies where the rate of miscarriage rose from about 25% in normal weight women to about 30% in overweight women. However the rate of miscarriage after spontaneously conceived pregnancies is also higher in overweight women (about 14% as compared to about 11%) – the miscarriage rates appear higher in IVF pregnancies because very early pregnancy loss is often missed in spontaneous pregnancies.

Furthermore, the higher the body mass index (BMI), the higher the miscarriage risk. When a woman’s BMI is greater than 25 (kg/m2) the chance of miscarriage after spontaneous pregnancy is about 12%, as opposed to about 11%. For a woman with a BMI greater than 30k (g/m2) it is about 14%, and for a BMI over 40 (kg/m2) it is about 17%.

What is not clear is why there is this association. Certainly polycystic ovary syndrome (PCOS) in overweight women has been associated with an increased risk of miscarriage – and the large studies that show an association between miscarriage and weight undoubtedly included overweight women with PCOS. Unfortunately, it is not known whether exercise and general health have a protective effect, although most healthcare workers would recommend increasing exercise and attempting weight loss because of the overall health benefits.

Overall, there is a small increase in the chance of miscarriage associated with being overweight. In your particular case, therefore, it is more likely that some other factors – most probably genetic abnormalities within the developing embryo – were responsible for the 2 miscarriages that you suffered. Certainly I would advocate your continuing your exercise and maintaining your general health. Furthermore, after 2 miscarriages it would be appropriate to be seen by a clinician with some expertise in the evaluation and treatment of recurrent miscarriage in order to exclude any other treatable causes.

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