Body Mass Index and the Risk of Recurrent Miscarriage

Dr. Randy Morris

Body Mass Index and the Risk of Recurrent Miscarriage

Dr. Randy Morris

Recurrent miscarriage is defined as 3 or more consecutive miscarriages . This condition affects 1% of fertile couples and in 50% of cases, the cause remains undiagnosed. Because of this, it remains one of the most distressing and stressful problems affecting couples trying to conceive.

Recently, it has been suggested that body mass index (BMI) plays a role in the risk of miscarriage in women with recurrent miscarriage.

Body Mass Index:

BMI is a measure of a person’s weight to height ratio. In this study, the World Health Organization’s standards of BMI were used to classify each woman. BMI was calculated using the formula: BMI ¼ weight/height2 (kg/m2).

Patients were first classified into two main groups:

>30 kg/m2 obese

< 30 kg/m2 non-obese

Subgroups were then created:

<19.0 kg/m2 underweight

19.0–24.9 kg/m2 normal weight

25.0–29.9 kg/m2 overweight

The obese group was also subdivided into two subgroups:

30.0–34.9 kg/m2 mild obesity

>35 kg/m2 moderate and severe obesity

Study Analysis:

A total of 844 pregnancies from 491 patients with recurrent miscarriage were included in the analysis.

The patients were all women referred to a recurrent miscarriage clinic. All pregnancies that occurred before referral to the clinic were excluded from evaluation.

Recurrent miscarriage was diagnosed from the occurrence of three consecutive spontaneous miscarriages before 20 weeks gestation. All subjects underwent investigations according to an established clinical protocol.

Outcomes Measured:

  • Miscarriage rates in all subgroups
  • Odds of miscarriage in the next pregnancy
  • Comparison of normal vs. underweight, normal vs. overweight, normal vs. obese patients


Obese and Underweight Patients at the Most Risk:

This study found that in women with recurrent miscarriage, a mild increase in the body mass index (overweight patients) did not increase the risk of miscarriage. However, obese and underweight patients have a small but significant increased risk of miscarriage in the subsequent pregnancy.

Normal vs. Underweight patients

Analysis of all pregnancies showed significantly increased odds of miscarriage in the underweight patients

Normal vs. Overweight patients

Analysis showed no increased odds of miscarriage. These patients were very similar.

Normal vs. Obese patients

Analysis of all pregnancies showed significantly increased odds of miscarriage in the obese patients

Maternal Age:

As expected, the most important factor predicting the occurrence of miscarriage was advanced maternal age. Somewhat surprising was that the next most important factor was an increased body mass index.

Neither the presence of PCOS (polycystic ovarian syndrome) nor the number of previous miscarriages significantly contributed to the model. This is an extremely important point. There is much misinformation about the risks of miscarriage in PCOS patients. Many patients are under the impression that PCOS patients have a higher risk of miscarriage. According to the results of this study, this does not appear to be true. While it is true that PCOS patients have a high rate of obesity, it is the obesity which is linked to miscarriage.

Underweight Patients:

Underweight BMI has also been associated with several adverse pregnancy outcomes, including maternal anemia, premature labor, and low birth weight. Only one study has previously suggested a possible increase in the risk of spontaneous miscarriage in this group of patients.


It should be emphasized that the effect of obesity and underweight on pregnancy outcomes in women with recurrent miscarriage is of a small magnitude. The presence of either of these factors should not stop an extensive search for other possible causes for recurrent miscarriage.