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BMJ. Nov 25, 2000; 321(7272): 1320–1321.
PMCID: PMC27536

Body mass and probability of pregnancy during assisted reproduction treatment: retrospective study

J X Wang, senior research officer, M Davies, senior research fellow, and R J Norman, professor of reproductive medicine

Being underweight or overweight has an adverse effect on reproduction.1,2 Overweight women have a higher incidence of menstrual dysfunction and anovulation, possibly because of altered secretion of pulsatile gonadotropin releasing hormone, sex hormone binding globulin, ovarian and adrenal androgen, and luteinising hormone and also because of altered insulin resistance. The prevalence of obesity in infertile women is high, but there is no conclusive evidence that extremes of weight are associated with a low rate of pregnancy in women receiving assisted reproduction treatment. This study examined whether body mass index (weight (kg)/(height (m)2)) is associated with reduced fecundity (the probability of achieving at least one pregnancy during treatment) in women receiving assisted reproduction treatment.

Participants, methods, and results

The participants were 3586 women who received assisted reproduction treatment between 1987 and 1998 in a tertiary medical unit in Adelaide, South Australia. Treatments included in vitro fertilisation (n=1972), intracytoplasmic sperm injection (n=1040), and gamete intrafallopian transfer (n=574). Patients underwent 8822 embryo transfer cycles. The overall implantation rate was 12.0%, and the clinical pregnancy rate was 24.1% in the study population. Causes of infertility included tubal blockage (34%), semen defects (35%), unexplained infertility (16%), and endometriosis (9%). Age of participants, treatment modalities, location of the treatment, number of embryos transferred, number of cycles of embryo transfer, and number of oocytes recovered were analysed to eliminate possible confounding effects. Polycystic ovarian syndrome was diagnosed, using normal criteria, in 25% (881/3586) of the women.3

Participants were stratified into five groups according to body mass index: “underweight” (<20), “moderate” (20.0-24.9), “overweight” (25.0-29.9), “obese” (30.0-34.9), and “very obese” ([gt-or-equal, slanted]35). Fecundity was defined as the probability of achieving at least one pregnancy throughout the treatment. Pregnancy was determined by ultrasonography of the embryonic sac (or sacs) in the womb at 4-6 weeks after embryo transfer. The clinical protocols have been described elsewhere.4 We compared the groups by using analysis of variance and a χ2 test. We assessed the effect of body mass index, controlling for the confounding factors, by logistic regression.

The number of treatment cycles and embryos transferred per cycle did not differ among the groups, but age varied significantly but unsystematically (table). There was a significant linear reduction in fecundity from the moderate group to the very obese group (P<0.001). The fecundity of the moderate group was almost 60% higher than that of the very obese group, and the fecundity of the underweight group was also significantly lower than that of the moderate group (P<0.05), indicating an “inverted U” relation between body mass index and fecundity.

Logistic regression analysis confirmed the independent effect of body mass on fecundity. When the significant effects of maternal age, number of embryos transferred, number of cycles received, treatment type, and cause of infertility were controlled for, the pregnancy rate among very obese women was half that of the moderate group. Polycystic ovarian syndrome had an independent effect on fecundity.

Commentary

A body mass index that was either high or low was associated with reduced probability of achieving pregnancy in women receiving assisted reproduction treatment. Mechanisms through which body mass affects reproduction that have been cited include menstrual disturbance and anovulation,5 but these problems can be overcome through assisted reproduction treatment. There is no evidence that body mass affects the quality of the embryo and therefore the pregnancy rate. We propose that other mechanisms, such as altered receptivity of the uterus after transfer of embryos or oocytes, possibly because of disturbed endometrial function, may cause reduced fecundity.

Table
Influence of body mass on probability of pregnancy during assisted reproduction treatment. Figures are values (SD) unless stated otherwise

Acknowledgments

We thank the staff of the Reproductive Medicine Unit in Adelaide for their contribution.

Footnotes

Funding: No additional funding.

Competing interests: None declared.

References

1. Correa H, Jacoby J. Nutrition and fertility: some iconoclastic results. Am J Clin Nutr. 1978;31:1431–1436. [PubMed]
2. Frisch RE. Body weight and reproduction [letter] Science. 1989;246:432. [PubMed]
3. Norman RJ, Masters SC, Hague W, Beng C, Pannall P, Wang JX. Metabolic approaches to the subclassification of polycystic ovary syndrome. Fertil Steril. 1995;63:329–335. [PubMed]
4. Norman RJ, Warnes GM, Wang X, Kirby CA, Matthews CD. Differential effects of gonadotrophin-releasing hormone agonists administered as desensitizing or flare protocols on hormonal function in the luteal phase of hyperstimulated cycles. Hum Reprod. 1991;6:206–213. [PubMed]
5. Lake JK, Power C, Cole TJ. Women's reproductive health: the role of body mass index in early and adult life. Int J Obes Relat Metab Disord. 1997;21:432–438. [PubMed]

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