Should IVF Be Delayed While Overweight Women Lose Weight?

Peter Kovacs, MD, PhD


November 23, 2015

The Effect of Mode of Conception on Obstetrical Outcomes Differs by Body Mass Index

Machtinger R, Zera C, Racowsky C, et al
Reprod Biomed Online. 2015;31:531-537

Obesity, Pregnancy, and Fertility

The prevalence of obesity in the western world has increased significantly in recent decades. According to the 2013 American College of Obstetrics and Gynecology Committee Opinion about obesity in pregnancy,[1] more than one third of women are obese, and more than half of pregnant women are overweight.

Obesity carries extra risks for both mother and newborn. Certain medical problems (hypertensive complications, gestational diabetes, thromboembolism, need for operative delivery, anesthesia-related problems, postoperative fever, and wound-related complications) become more common as body weight increases.

Infertility, especially related to ovulatory dysfunction, is more common among obese women. Obese women are more resistant to stimulation and therefore need more medications for stimulation, have lower pregnancy rates after in vitro fertilization (IVF), and have a higher risk for miscarriage.[2]

The use of IVF itself, even in the case of singletons, is associated with increased risk for prenatal medical complications, preterm delivery, low birth weight, and small for gestational age newborns.[3]

In this study, Machtinger and colleagues assessed how body mass index (BMI) modifies the risks associated with a singleton pregnancy when conceived spontaneously vs through IVF.

Study Summary

The analysis is based on retrospective data from 1171 spontaneously conceived and 464 IVF singleton pregnancies. In addition to demographic characteristics, data were collected for ischemia-related or hypertensive complications, gestational diabetes, preterm delivery, preterm rupture of membranes, mode of delivery, and birth weight in the index pregnancy. The prevalences of these pre-, and perinatal complications were compared between spontaneously conceived and IVF pregnancies in three BMI categories (normal: 18.5-24.9 kg/m2, overweight: 25-29.9 kg/m2, and obese: >30 kg/m2). Maternal age, parity, race, smoking status, and prepregnancy medical problems were controlled for in the analysis.

Among normal-weight women, more problems related to placental ischemia were found in IVF pregnancies (odds ratio, 2.24; 95% confidence interval, 1.25-4.04), but other problems (pre-eclampsia, gestational diabetes, small for gestational age infant, preterm delivery, premature rupture of membranes) were not associated with the mode of conception. Among overweight and obese women, none of these medical complications were affected by the mode of conception.

The mode of delivery (Cesarean section vs vaginal) did not differ between IVF or spontaneous pregnancies in any of the BMI groups.

The mean neonatal birth weight was lower following IVF pregnancies in women with normal BMI (2799.8 g vs 2961.3 g) and in overweight women (2950.7 g vs 3194.4 g). Among obese women, however, the mean birth weight did not differ between spontaneously conceived vs IVF-conceived pregnancies (3122 g vs 3127.2 g). The incidence of low birth weight was increased in pregnancies conceived by IVF in normal weight and overweight women. Among obese women, no difference in low birth weight rate was seen on the basis of mode of conception. It was concluded that the adverse impact of assisted reproduction technology (ART) on perinatal outcome is masked by the adverse impact of obesity.


Differences in pregnancy outcomes with ART may be related to the treatment itself (use of stimulation, in vitro culture), but they can also result from differences between the fertile and infertile populations. It is known that hypertensive complications, gestational diabetes, low birth weight, placental abruption, premature rupture of membranes, preterm delivery, and neonatal intensive care admissions are more common in IVF pregnancies.[3]

Obese women are more likely to be infertile, primarily owing to problems with ovulation. They will more often require some form of ART to conceive, and they tend to have poorer responses to the treatment.[2] Stimulation becomes more challenging—some obese women will hyperrespond to treatment, but most will be resistant to stimulation. Obesity has an adverse effect on implantation, and therefore pregnancy rates will be lower and miscarriage rates will be higher. Even if the pregnancy progresses through the first trimester, the patient still faces second and third trimester complications of obesity.

This study found mostly similar outcomes between spontaneous and IVF-conceived pregnancies in three different BMI categories. Some differences in the incidence of placental ischemia-related problems and birth weight were observed in normal and overweight women. Among obese women, however, pregnancy outcomes did not differ on the basis of mode of conception. The investigators speculated that among obese women, the problems related to obesity are so extensive that the adverse impact of IVF no longer modifies the outcomes.

This is important information because the goal of ART is to help as many women as possible to have successful pregnancies, defined as the full-term delivery of a healthy child. Therefore, when an overweight or obese patient is seen for preconception counseling, the importance of achieving and maintaining normal BMI must be stressed. These couples need to be educated about proper lifestyle modifications, healthy diet, and the importance of regular exercise. Fertility treatment could even be temporarily withheld to allow time for the lifestyle changes to have an effect. The patient should be counseled that rather than rushing treatment and possibly failing, she should get into better physical shape first. In the long run, she will not only increase her chance to have a successful pregnancy but will save money as well if treatment efficacy improves. It also would be helpful for further studies to assess the relationship between adverse perinatal outcomes and BMI in IVF patients, with particular interest in the highest BMI (>35 kg/m2 or even >40 kg/m2) populations.


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