We read with interest the paper by Dr. Eogan and colleagues regarding differences in method of delivery when the fetus is male as compared to female.1 In a study of 2439 term nulliparous women with spontaneous labor (the same selection criteria used by Eogan et al.), we also observed a higher rate of cesarean when the fetus was male as compared to female (13.2% male, 9.6% female; OR=1.4, 95% CI=1.1, 1.8).2 The nearly identical relative increase in cesareans reported in our study (40%) and Eogan et al. (50%) in the face of vastly different baseline cesarean rates (5% National Maternity Hospital, 11% Brigham and Women’s) suggests a biologic basis for the finding. In our population, cesareans for failure to progress were increased by 30%, but that increase was explained by the larger size of male fetuses, in particular their larger head circumference. Cesareans for non-reassuring fetal status also increased when the fetus was male and the association remained when adjusted for gestational age and fetal size (adjusted OR=2.2, 95% CI=1.3, 4.0). This is consistent with the Eogan et al. finding of an increase in fetal blood sampling when the fetus was male.

While the reason for the increased diagnosis of non-reassuring fetal status in the presence of a male fetus is unclear, it may reflect differences in development between male and female fetuses. There are non -human data suggesting that the sympathoadrenal system develops earlier in female fetuses.3 In humans, sex differences in catecholamine levels at birth have been noted in preterm fetuses, with females producing more catecholamines in response to hypoxia.4 It has been postulated that this catecholamine surge improves the ability of the fetus to withstand hypoxia. This is supported by Lagercrantz, who found that among infants with moderate acidosis, those with Apgar scores<_7 had="had" lower="lower" levels="levels" of="of" catecholamines="catecholamines" at="at" birth="birth" than="than" those="those" infants="infants" with="with" higher="higher" apgar="apgar" scores.5="scores.5" in="in" our="our" study="study" we="we" also="also" found="found" that="that" male="male" fetuses="fetuses" delivered="delivered" by="by" cesarean="cesarean" section="section" for="for" fetal="fetal" distress="distress" scores="scores" female="female" indication.2="indication.2" while="while" further="further" research="research" is="is" needed="needed" to="to" elucidate="elucidate" the="the" mechanisms="mechanisms" responsible="responsible" sex="sex" differences="differences" noted="noted" both="both" eogan="eogan" et="et" al.="al." and="and" group="group" it="it" plausible="plausible" such="such" relate="relate" catecholamine="catecholamine" responses="responses" during="during" labor.="labor." p="p"/> Ellice Lieberman, MD, DrPH
David Acker, MD
Department of Obstetrics and Gynecology, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA

Fredric Frigoletto, MD
Department of Obstetrics and Gynecology, Massachusetts General Hospital, Harvard Medical School, Boston, MA

1. Eogan, MA, Geary, MP, O"Connell, MP,Keane, DP, Effect of fetal sex on labour and delivery:retrospective review. BMJ, 2003. 326: p. 137.

2. Lieberman, E, Lang, JM, Cohen, AP, Frigoletto, FD, Jr., Acker, D,Rao, R, The association of fetal sex with the rate of cesarean section [see comments]. Am J Obstet Gynecol, 1997. 176(3): p. 667-71.

3. Padbury, JF, Hobel, CJ, Lam, RW,Fisher, DA, Sex differences in lung and adrenal neurosympathetic development in rabbits. Am J Obstet Gynecol, 1981. 141(2): p. 199-204.

4. Greenough, A, Lagercrantz, H, Pool, J,Dahlin, I, Plasma catecholamine levels in preterm infants. Effect of birth asphyxia and Apgar score. Acta Paediatr Scand, 1987. 76(1): p. 54-9.

5. Lagercrantz, H, Ashpyxia and the Apgar score. Lancet, 1982. (8278): p. 966.

Competing interests:   None declared

The paper on fetal sex and mode of delivery by Eogan et al.(18/1/03) is interesting but hardly original.With Carr-hill,I published in this journal in 1982(Hall and Carr-hill,1982) the observation that instrumental delivery and emergency Caesarean section were more likely when the fetus was male. No doubt Eogan et al's ignorance of this stems from the fact that their Medline search was restricted to the years 1996-2002.

Does this mean that knowledge can be re-invented every 10 years or so? Or should authors aspire to a more thorough literature search?

Marion Hall

1.Hall M H,Carr-Hill R The impact of sex ratio on onset and management of labour. BMJ 1982 285 401-403

Competing interests:   None declared