O-004 Male and female blastocysts display differences in development such that embryos assigned an identical morphological grade may have differing viability dependent on their sex
Authors: Kilbee M , Del Canto Cano A , Clark G , Coudereau C , Fernandez Marcos E , Babariya D , Wells D 
Published: June 2023

Study question

Embryo selection methods based upon morphological or morphokinetic evaluation assume that male and female embryos have identical rates of preimplantation development, but is this true?

Summary answer

The distribution of trophectoderm grades differs for male and female blastocysts. Furthermore, male and female embryos of identical grade may have different probabilities of viability.

What is known already

Previous studies have suggested that male and female embryos may have subtle differences in their rates of preimplantation development. However, this possibility remains controversial. Apart from being of scientific interest, the question of whether the sex of an embryo can affect its growth trajectory is of clinical importance. Morphological grading, the primary method used by most IVF clinics when deciding which embryo to prioritise for transfer, assumes that developmental rates are independent of sex. Similarly, morphokinetic strategies for embryo evaluation, using data gathered from time-lapse incubators, are likely to be compromised if male and female embryos have differing developmental behaviour.

Study design, size, duration

1,241 blastocysts underwent PGT-A and were shown to be euploid. Chromosome analysis also revealed the sex of the embryos, although this was not disclosed to patients. Standard morphological grading was carried out blindly with respect to the sex of the embryo. Information on clinical outcomes following embryo transfer was available for a subset of 336 embryos. Data was evaluated using various statistical methods to reveal any associations between embryo sex, morphology and clinical outcome.

Participants/materials, setting, methods

Embryos included in this study were derived from patients undergoing routine IVF with PGT-A. The only embryos excluded were those derived from patients carrying a monogenic disease mutation or a chromosome rearrangement. Embryos underwent PGT-A at the blastocyst stage on either day-5 or day-6, using a highly validated method. The euploid embryos were divided into male and female groups and blastocyst morphological grades were considered with respect to rates of implantation, miscarriage and ongoing pregnancy/birth.

Main results and the role of chance

The proportion of embryos biopsied on day-5 versus day-6 was identical for males and females (68% day-5 for both). No difference was observed in blastocyst expansion or inner cell mass grading on day-5. However, a highly significant difference was noted in the distribution of trophectoderm grades (A, B, C, D according to the system of Gardner and Schoolcraft, 1999) (P &lt; 0.0001). This was characterised by a disproportionate representation of the highest grade amongst male embryos. 21.6% of male blastocysts had trophectoderm graded ‘A’, compared to 14.9% of females. Interestingly, euploid male embryos with a ‘B’ grade trophectoderm were associated with significantly higher implantation rates than females of the same morphological grade (82.1% vs. 58.7%; P = 0.0002), but the males of this grade also experienced a higher incidence of biochemical losses (16.7% vs. 4.2%; P = 0.017). As a group, male embryos do not have greater viability than female embryos, which implies that the greater proportion of high trophectoderm grades amongst these embryos is not an indicator of superior potential. In turn, this suggests that different criteria should be used for grading trophectoderm in males and females.

Limitations, reasons for caution

It may be desirable to introduce parallel morphological grading systems, one for male embryos and another for female. However, this would only be applicable in cycles involving PGT-A. The observation that embryos of equal grade can have different outcomes depending on their sex should be confirmed in a prospective study.

Wider implications of the findings

These results suggest the development of sex-specific morphological grading strategies might provide more reliable insights into embryonic potential, increasing the likelihood of selecting a viable embryo for transfer. Consideration of differences in the development of male and female embryos will also be important when developing morphokinetic algorithms for embryo selection.

Trial registration number

Not applicable