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Sir,

We thank Cahill and Wardle for their interest in our paper and wish to clarify the issues raised in their letter. The cancellation rate in our study was 8.1%. This is consistent with other cancellation rates reported in the literature, particularly when a large number of cycles is considered. For example, the cancellation rate reported in the North American 1994 database (26,551 cycles)1 was 13.2%. Perhaps an explanation of the low cancellation rate reported by Cahill et al.2 (1.7%) was the fact that they studied a relatively small number of cycles, and should they study a larger number then the true cancellation rate would become apparent– the well known ‘regression to the mean’ phenomenon.

Secondly, they enquire in their letter whether we did ‘take account of sperm dysfunction which … may have adversely affected fertilisation rates disparately between … subgroups’. Yes we did, as we stated in our paper (‘results’ section, fourth paragraph, last three lines). To reiterate, there was no difference in the fertilisation rate whether all couples were considered or only those with no male factor infertility.

Finally, Cahill and Wardle enquire in their letter about the effect of the number of embryos transferred on our main conclusions. Those conclusions were that patients with abnormal basal FSH had about a six-fold increase in the cancellation rate compared with those with normal levels (31.2% versus 5.5%). They also had, on average, half the number of oocytes collected (12 versus 6). Both these conclusions have nothing to do with the numbers of embryos transferred. Controlling for this when reaching those conclusions might have been statistically interesting, but of little clinical significance.

The number of embryos transferred is well known to affect the pregnancy rate. Had we found that FSH–independent of age– was significantly associated with pregnancy rate, then we would have analysed the results controlled for the number of embryos transferred. As we have found that it was not, then performing that analysis was not necessary. Here again, we were interested mainly in the external validity (clinical application) of our study, rather than just its internal validity.

We agree with them, of course, that larger studies are necessary to clarify the effect of basal FSH levels on the final outcome of IVF treatment, the live birth rate.

References

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  2. References
  • 1
    The American Fertility Society. Assisted reproductive technology in the United States and Canada: 1994 results generated from the American Society for reproductive Medicine/Society for Assisted Reproductive Technology Registry. Fertil Steril 1996; 66: 697705.
  • 2
    Cahill DJ, Prosser CJ, Wardle PG, Ford WCL, Hull MGR. Relative influence of serum follicle stimulating hormone, age, and other factors on ovarian response to gonadotrophin stimulation. Br J Obstet Gynaecol 1994; 101:9991002.