Both high and low BMI increase the risk of miscarriage after IVF/ICSI and FET.
Veleva Z, Tiitinen A, Vilska S, Hydén-Granskog C, Tomás C, Martikainen H, Tapanainen JS.
Department of Obstetrics and Gynecology, University of Oulu, PO Box 5000, Oulu FIN-90014, Finland.
Hum Reprod. 2008 Feb 15

BACKGROUND The extremes of BMI are associated with an increased risk of miscarriage both in spontaneously conceived pregnancies and after fertility treatment. The aim of the present study was to study the effect of BMI on miscarriage rate (MR) in fresh IVF/ICSI, and in spontaneous and hormonally substituted frozen-thawed embryo (FET) cycles.
METHODS Analysis was carried out on 3330 first pregnancy cycles, performed during the years 1999-2004, of which 2198 were fresh, 666 were spontaneous and 466 were hormonally substituted FET cycles. A categorical, a linear and a quadratic models of the effect of BMI on miscarriage were studied by logistic regression. Factors related to patient characteristics, protocol and embryo parameters were also examined.
RESULTS MR was higher in hormonally substituted FET (23.0%), compared with the fresh cycles (13.8%) and spontaneous FET (11.4%, P < 0.0001). Multivariate logistic regression revealed that the relationship between BMI and the risk of miscarriage is not linear but quadratic (U-shaped) (P = 0.01), indicating a higher risk of miscarriage in underweight and obese women. Hormonal substitution for FET was also associated with a 1.7-fold higher MR, compared with the fresh cycles (P = 0.002, 95% confidence interval 1.2-2.3).
CONCLUSIONS Obese and underweight women have an increased risk of miscarriage, and hormonally substituted FET is associated with an even higher MR.

Comments by Dr.Amos
This study again confirms that body weight plays an important part in getting pregnant and having a healthy pregnancy.
Before trying to conceive, you should first try and get to your optimal body weight (BMI)

Calculate your BMI HERE