Intended for healthcare professionals

Rapid response to:


Time to pregnancy and sex of offspring: cohort study

BMJ 2005; 331 doi: (Published 15 December 2005) Cite this as: BMJ 2005;331:1437

Rapid Response:

Entrenched misinformation about X- and Y-sperm

Entrenched misinformation about X- and Y- sperm

In a recent paper, Smits et al1 found that the longer it took a woman
to become pregnant the more likely it was that she would have a male
infant. While the methodology and data are impressive, the authors’
explanation for their findings is less so, resting as it does on the claim
that Y-chromosome-bearing spermatozoa (Y-sperm) swim faster than X-
chromosome-bearing spermatozoa (X-sperm).

This widely-held idea appears to have originated with Dr Landrum B
Shettles. In 1960, using his new phase-contrast microscope, Shettles
claimed to have observed “two distinct populations”2 of spermatozoa.
After attempting to count the chromosomes, he concluded that “the smaller
heads contain the Y- and the larger the X-chromosome.” “There were no
intermediate types.”2. The following year he reiterated these findings,
adding that “… smaller headed spermatozoa can migrate more rapidly and
fertilize the ovum more often in the distal part of the tube”.3

Reading Shettles’ reports in Nature and other peer-reviewed journals,
many researchers thereafter believed that Y-sperm swim faster than X-
sperm. The finding particularly influenced research on sperm separation.
In an article also published in Nature, researchers noted that the basis
of their procedure was “the superior swimming ability of Y sperm as a
function of time”. 4

Although there were several attempts to correct this impression 5,6
it was not until the development of computer-assisted sperm analysis
(CASA)7 that reliable observations could be made. So far, researchers
have found no morphological differences between human X- and Y- sperm 8,.
“Neither mature sperm nor their precursors possess significant
morphological differences between X and Y genotypes”8 and “Y bull sperm do
not swim faster than X sperm” 9.


1. Smits LJM, de Bie RA, Essed GG, van den Brandt PA. Time to
pregnancy and sex of offspring: cohort study. BMJ 2005; 331:1437-8.

2. Shettles LB. Nuclear morphology of human spermatozoa. Nature

3. Shettles LB. After office hours: Conception and birth sex ratios:
A review. Obstet Gynec 1961;18: 122-130.

4. Ericsson R J, Langevin C N, Nishino M. Isolation of fractions rich
in human Y sperm. Nature 1973; 246: 421-4.

5. Rothschild. X and Y spermatozoa. Nature 1960; 187:253-4.

6. Gledhill BL. Selection and separation of X- and Y-chromosome-
bearing mammalian sperm. Gamete Res 1988; 20:377-95.

7. Moruzzi JF, Wyrobek AJ, Mayall BH, Gledhill BL. Quantification and
classification of human sperm morphology by computer-assisted image
analysis. Fertil Steril 1988; 50:142-52.

8. Hossain AM, Barik S, Kulkarni PM. Lack of significant
morphological differences between human X and Y spermatozoa and their
precursor cells (spermatids) exposed to different prehybridization
treatments. J Androl 2001; 22:119-23.

9. Penfold LM, Holt C, Holt WV, Welch DG, Cran DG, Johnson LA.
Comparative motility of X and Y chromosome-bearing bovine sperm separated
on the basis of DNA content by flow sorting. Mol Reprod Dev 1998; 50:323-

Competing interests:
None declared

Competing interests: No competing interests

16 March 2006
Valerie J. Grant
senior lecturer
University of Auckland, Private Bag 92019, Auckland, 1. New Zealand,