Birthweight and Sex Ratio after Blastocyst Transfer

Article Title: “Birth weight and sex ratio after transfer at the blastocyst stage in humans”

Authors and Affiliations: Yves J.R. Ménénzo, Institut Rhonalpin, Fondation Marcel Mérieux, Bron, France; Jacques Chouteau, Clinique Belledone, Saint-Martin-d’Hères, France; M.J. Torelló, Institut Universitari Dexeus, Barcelona; Ann Girard, Clinique Belledone; and Anna Viega, Universitari Dexeus.

Journal: Fertility and Sterility, Volume 72, Number 2, pp.221-224.

Summarized by Christine M. Schroeder, Ph.D.

During selection of embryos for IVF transfer, preference is given to those that cleave more rapidly. In some mammals, research has shown that male embryos have higher initial cleavage rates than female embryos, and that male embryos are thus more likely to be the first to achieve the blastocyst stage. Therefore, it is possible that blastocyst selection procedures in IVF will lead to a higher than expected proportion of male blastocysts being transferred.

Additional research in animals has revealed a higher incidence of overweight males, some with abnormalities, when IVF is used in conjunction with co-culture. It is not clear, however, whether either the potential sex ratio difference or the incidence of overweight males applies to the human population.

The current study examined both the birth weight and the sex ratio of infants who were born after the use of blastocyst transfer. Patients came from three IVF centers, and the infants studied were from three groups1:

  1. The fresh transfer group: Infants who were born following fresh blastocyst transfer with coculture
  2. The frozen transfer group: Infants born following frozen blastocyst transfer
  3. The comparison group: Infants born after spontaneous conception at a university hospital over a seventeen month period

The results indicated that:

  • The fresh blastocyst group had a significantly higher proportion of males (58.3 percent) than females (41.3 percent)
  • The frozen transfer blastocyst group did not have significantly different proportions of males (51.9 percent) and females (48.1 percent)
  • Male babies in the fresh transfer group had an average birth weight of 2,788 grams, compared to a significantly higher average weight of 3,218 grams in the comparison group (see however, the note regarding birth weight measures at the end of the summary)
  • Male babies in the frozen transfer group also had an average birth weight (3,040 grams) significantly lower than that of the comparison group

The researchers concluded that human male embryos do indeed cleave more rapidly in the preimplantation stage than female embryos. As a result, they reach the blastocyst stage sooner and are more likely to be selected for fresh transfer. In contrast, frozen blastocysts transfers were not subject to the same selection criteria – embryos that are frozen general are generally slower-growing and are not subject to the same selection criteria as fresh transfer. As a result, births resulting from frozen transfers did not have a sex ratio significantly different from the spontaneously conceived control group. Finally, blastocyst co-culture does not seem to have the same negative effects in humans that they do in some mammals.

Note: This study is interesting, but I think the authors neglected one important factor. The mean birth weights that were analyzed are a combination of babies from both single and multiple pregnancies. Although there is no mention of the fact, it would be reasonable to assume that there was a higher rate of multiple pregnancies in one or both of the blastocyst groups. Since babies from multiple pregnancies usually weigh less than singletons, the higher proportions of multiples in the blastocyst groups very well might have dragged down the average weight of the entire group.
Looking at the average weights they listed for singleton-only pregnancies convinced me even further that the differences might be related to multiples. Among male babies born as singletons, the average weight for the fresh transfer group was 3,191 grams, the average weight for the frozen transfer group was 3,333 grams, and the average weight for the comparison group was 3,308 grams. It is doubtful that these differences would be statistically significant. A similar pattern was also found for female singleton babies.

1 There was also a group of infants born after use of sequential media, but the group was much smaller and did not offer the same statistic power for analyses. The analyses of this group were presented in the paper, but are not included in this summary.

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