Correspondence

The Biology of Human Sex Differences

N Engl J Med 2006; 355:98July 6, 2006

Article

To the Editor:

In the discussion of biologic differences between male and female fertility (April 6 issue),1 Federman states that women are fertile for only 12 hours each month. Although the egg is viable for 12 hours or less, the window of fertility in women is approximately five to six days in each menstrual cycle,2-4 depending on the presence of estrogenic cervical mucus that maximizes the storage, survival, and transport of sperm until ovulation.4,5

Joseph B. Stanford, M.D., M.S.P.H.
University of Utah, Salt Lake City, UT 84108
jstanford@dfpm.utah.edu

5 References

1. 1

   Federman DD. The biology of human sex differences. N Engl J Med 2006;354:1507-1514
   Full Text | Web of Science | Medline

2. 2

   Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation -- effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med 1995;333:1517-1521
   Full Text | Web of Science | Medline

3. 3

   Colombo B, Masarotto G. Daily fecundability: first results from a new data base. Demogr Res 2000;3:39-39
   CrossRef | Medline

4. 4

   Stanford JB, White GL, Hatasaka H. Timing intercourse to achieve pregnancy: current evidence. Obstet Gynecol 2002;100:1333-1341
   CrossRef | Web of Science | Medline

5. 5

   Bigelow JL, Dunson DB, Stanford JB, Ecochard R, Gnoth C, Colombo B. Mucus observations in the fertile window: a better predictor of conception than timing of intercourse. Hum Reprod 2004;19:889-892
   CrossRef | Web of Science | Medline

Author/Editor Response

Stanford rightly calls attention to the elegant estrogen-dominated events that precede ovulation and that favor passage of sperm through the cervix and to a point of potential fertilization. He is also right to call this a “window of fertility,” because sperm deposited during this time are viable longer than are eggs (48 to 72 hours) and can thus be on site when ovulation occurs. My wording that a woman is fertile for only 12 hours a month referred to the viability of the egg and its potential for fertilization, and Stanford does not appear to disagree with that.

Daniel D. Federman, M.D.
Harvard Medical School, Boston, MA 02115