Twins Born Following Fertility Treatment: Implications for Quantitative Genetic Studies

The impact of specific fertility treatments on cognitive development in childhood and adolescence: a systematic review

STUDY QUESTION

Does fertility treatment influence cognitive ability in school aged children, and does the impact vary with the type of treatment?

SUMMARY ANSWER

The available high-quality evidence indicates that specific treatments may give rise to different effects on cognitive development, with certain treatments, including ICSI, associated with cognitive impairment.

WHAT IS KNOWN ALREADY

Previous reviews of the literature concerning cognitive outcomes among children conceived with medical assistance have concluded that study findings are generally 'reassuring', but limited attention has been paid to the quality of this research. In addition, no review has separately assessed the range of treatment modalities available, which vary in invasiveness, and thus, potentially, in their effects on developmental outcomes.

STUDY DESIGN, SIZE, DURATION

A systematic review was undertaken. We searched PubMed, PsycINFO and the Educational Resources Information Centre database to identify English-language studies published up until 21 November 2016.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Two authors independently reviewed identified articles, extracted data and assessed study quality. Studies were eligible if they assessed cognitive development from age 4 years or more, among children conceived with fertility treatment compared with either children conceived naturally or children born from a different type of fertility treatment. Where available, data were extracted and reported separately according to the various components of treatment (e.g. mode of fertilization, embryo freezing, etc.). Risk of bias was assessed using the Newcastle-Ottawa Scale, with a score ≥7/9 indicative of high quality.

MAIN RESULTS AND THE ROLE OF CHANCE

The search identified 861 articles, of which 35 were included. Of these, seven were rated high quality. Most studies (n = 22) were subject to selection bias, due to the exclusion of children at increased risk of cognitive impairment. Among high-quality studies, there was no difference in cognitive outcomes among children conceived with conventional IVF and those conceived naturally. Findings among high-quality studies of children conceived with ICSI were inconsistent: when compared with children conceived naturally, one study reported lower intelligence quotient (IQ; 5-7 points, on average) among ICSI children whereas the remaining two high-quality studies reported no difference between groups. Furthermore, among the three high-quality studies comparing children conceived with ICSI compared with conventional IVF, one reported a significant increase in the risk of mental retardation, one reported a small difference in IQ (3 points lower, on average) and one no difference at all. There were scant studies examining exposure to embryo freezing, or less invasive treatments such as ovulation induction without IVF/ICSI.

LIMITATION, REASONS FOR CAUTION

Most existing studies had methodological limitations including selection bias and/or failure to address confounding by family background. In addition, a meta-analysis could not be performed due to heterogeneity in the assessment of cognitive outcomes. These factors impeded our ability to synthesize the evidence and draw reliable conclusions.

WIDER IMPLICATIONS OF THE FINDINGS

The conflicting findings among studies of children conceived with ICSI require clarification, in light of the increasing use of this technique for reasons other than male-factor infertility. Further population-based studies are needed that utilize contemporary data to examine specific aspects of treatment and combinations of techniques (e.g. ICSI with frozen embryo cycles). Importantly, studies should include the complete group of children exposed to treatment.

STUDY FUNDING/COMPETING INTEREST(S)

A.R.R. is supported by a Career Development Fellowship from the National Health and Medical Research Council of Australia. L.J.M. is funded by a fellowship from the Heart Foundation of Australia. The authors declare there are no competing interests.

TRIAL REGISTRATION NUMBER

Not applicable.

How does the inclusion of twins conceived via fertility treatments influence the results of twin studies?

Rates of twinning have risen dramatically over the last 30 years, from 1 in 53 births in 1980 to 1 in 30 births in 2009 (Martin et al. (January 2012). Three decades of twin births in the United States, 1980-2009. Atlanta, GA: Centers for Disease Control and Prevention, National Center for Health Statistics). This increase is largely attributable to increases in the use of fertility treatments (i.e., ovulation induction and in vitro fertilization) combined with delays in parenthood. Although this increase means that more twins are available for recruitment into twin studies, it also has potential consequences for the heritability estimates obtained in these studies. This study sought to evaluate this possibility, making use of the ongoing Michigan Twins Project (N = 7,261 families with twins aged 3-17 years), an arm of the Michigan State University Twin Registry. Results revealed that, on average, twins conceived via fertility treatments had lower rates of behavior problems than those conceived naturally, although these behavioral differences could be explained largely by demographic and socio-economic differences across the two types of twin families. Twin similarity did not meaningfully differ across fertility treatment status. We thus conclude that estimates of genetic and environmental influences obtained from twin studies over the last 10-15 years are more or less unaffected by the inclusion of twins conceived via fertility treatments in their samples.

Examining differences in psychological adjustment problems among children conceived by assisted reproductive technologies

The aim of this study was to examine whether there was variation in levels of psychological adjustment among children conceived through Assisted Reproductive Technologies using the parents’ gametes (homologous), sperm donation, egg donation, embryo donation and surrogacy. Information was provided by parents about the psychological functioning of 769 children aged 5 to 9 years who had been born using ART (from the five groups described). Comparisons were made between the different conception groups, to UK national norms and, for a sub-sample of multiple births, to an age-matched twin sample. No differences were found between the conception groups except that fathers from the egg donation group rated children higher in conduct problems compared to other ART groups. No effects were observed by ART treatment type (ICSI vs. IVF, GIFT and IUI). There was some evidence of lower conduct problems and prosocial behaviour among children conceived through homologous IVF compared to national norms. Taken together, however, consistent differences between groups and in comparison to naturally conceived children were not apparent for mother- or father-rated adjustment problems. Children conceived with assisted reproductive technologies, regardless of whether they are genetically related or unrelated to their parents or born by gestational surrogacy do not differ in their levels of psychological adjustment. Nor do they appear to be at greater risk of psychological adjustment problems in middle childhood compared to naturally conceived children.

The Cardiff Study of all Wales and North West of England Twins (CaStANET): a longitudinal research program of child and adolescent development

The study of twins and their families provides a highly useful tool for disentangling the genetic and environmental origins of traits. The Cardiff Study of All Wales and North West of England Twins (CaStANET) has followed children and adolescents over time into early adulthood, assessing a wide range of aspects of behavior and psychopathology using self-, parent and teacher reports. Four main waves of data collection have taken place to date, which have provided a wealth of information on the contributions of genetic and environmental risk factors to the psychological health of young people. This article first describes the CaStANET register and subsequently presents some of the findings that have emerged from this resource, with a focus on depression and anxiety, chronic fatigue, attention-deficit/hyperactivity disorder, conduct problems and prosocial behavior. We describe in somewhat more detail the 4th wave of data collection, which has recently been completed and has provided us with extensive information on substance use and problem use as well as associated risk factors in the twins and their families, including longitudinal data on conduct problems and the relations between family members. Because of the wealth of data already collected and the opportunity for genetically informative analyses over time, CaStANET provides a valuable resource for understanding the complexities of the psychological development of young people.