Endometrial thickness in modified natural-cycle frozen-thawed embryo transfer cycles: Should we ignore the red traffic lights?

Sir,
Groenewoud et al recently concluded that endometrial thickness does not predict fertility success in modified natural-cycle frozen-thawed embryo transfer (mNC-FET) cycles.1 They suggest that embryo transfer can be planned regardless of endometrial thickness. The authors find an area under the receiver operating characteristics curve of 0.5 (Figure 3, P = .5) and pregnancy rates that do not statistically significantly differ depending on endometrial thickness, but we want to challenge this conclusion.

In fact, the ongoing pregnancy rates in women with an endometrium < 7 mm are 4.9% compared with 13.4% in women with thicker endometrium. A lack of statistical significance does not allow denial of this effect. The authors, however, state that “until larger cohort studies are available, the present data indicate that canceling cycles solely on the basis of endometrial thickness may not be indicated in mNC-FET”

We beg to differ. Let us compare this to traffic safety. Traffic lights predict the risk of collision at an intersection, and we would advise not to ignore them if the results between traffic light color and collision were not statistically significant. However, if we apply the traffic light analogy to the conclusion drawn by Groenewoud et al, it would read: “In conclusion, based on the results of the present study, crossing an intersection by car drivers can be planned irrespective of traffic light color. In drivers that approach the intersection while the lights are red, the risks of collision seem to be higher. Both conclusions should be confirmed in a larger cohort study. Until then, the present data indicated that stopping at an intersection on the basis of the color of the traffic light may not be indicated while driving a car.”

Use of endometrial thickness as a decision tool is obviously also dependent on its cost-effectiveness and on the question of whether a thin endometrium is an incidental finding, or one that is likely to recur in subsequent cycles. However, based on the present data, we feel that women with an endometrial thickness < 7 mm should be given the option to delay transfer.