Folia primatol. 31: 246-250 (1979)

Fetal Sex Ratio in the Rhesus (Macaco mulatto) R.F. DiGiacomo and P. W. Shaughnessy1 Department of Epidemiology, School of Public Health and Community Medicine, University of Washington, Seattle, Wash., and Department of Preventive Medicine, School of Medicine, University of Colorado Medical Center, Denver, Colo.

Key Words. Rhesus • Fetus • Sex ratio Abstract. Fetal sex ratio was determined in several hundred timed Macaca mulatto pregnancies terminated by hysterotomy between 75 and 179 days’ gestation. From 75 to 149 days’ gestation a greater number of females occurred in utero, while after 150 days a greater number of males was observed. However, this difference was not statistically significant.


The in utero fetal sex ratio for the rhesus has not been reported previously. The sex ratio at various gestational intervals was derived using data from approximately 850 fetuses delivered by hysterotomy at gestational ages of 75-179 days.

Materials and Methods Fetal sex ratio was determined from 855 Macaca mulatto pregnancies terminated by cesarean section at the Laboratory of Perinatal Physiology, National Institute of Neu­ rological Diseases and Stroke, San Juan, P.R. All pregnancies were the result of timed matings in which the female was exposed to the male for a 48-hour period during the middle of her menstrual cycle. Pregnancy was determined about 30 days later by the method of

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1 The authors gratefully acknowledge the expert technical assistance of Jeanne Smith.


Rhesus Fetal Sex Ratio

bimanual palpation of the uterus per rectum. Fetuses were delivered by hysterotomy between 75 and 179 days’ gestation. Sex was determined by examination of the external genitalia in live singleton fetuses [DiGiacomo et al., 1978).


Prior to 150 days’ gestation, the male/female ratio was 0.89 indicating a female excess while after 150 days the sex ratio was 1.09 indicating a male excess (table I). However, this difference was not statistically significant using Fisher’s exact test (p = 0.17).


The sex ratio at term is determined by two factors, the sex ratio at conception and the sex differential of reproductive wastage during pregnancy. Park [1957] determined that nuclear sexing of trophoblastic cells could be performed after the 18th day in his examination of 18 9- to 34-day-old rhesus embryos. Of the 5 embryos 18-34 days old, 3 were males and 2 were females, too few to derive a sex ratio for the embryonic period. Valerio et al. [1969] reported a female excess in abortions and stillbirths in both imported pregnant and laboratory-bred rhesus. More recently it has been shown that fetal mortality after 130 days’ gestation is greater for females than males [Shaughnessy et al., in press]. However, after 150 days’ gestation in utero (table I) and at term delivery in the rhesus there is an excess of males [Shaughnessy et al., in press]. Assuming that the female excess begins in embryonic life this suggests that more females than males are conceived. The female excess in fetal mortality

Gestational age, days



Male/female ratio

75-129 130-149 150-159 160-169 170-179 75-179

22 36 251 111 20 440

27 38 243 92 15 415

0.81 0.95 1.03 1.21 1.33 1.06 Downloaded by: King's College London - 8/16/2018 5:30:58 AM

Table I. Rhesus fetal sex ratio by gestational age at cesarean section













throughout gestation may merely reflect the fetal excess in utero since there are no known selective mortality factors for sex during gestation. The lower limit of fetal survival outside the uterus is about 130 days’ gestation [Shaughnessy et al., in press]. The distribution of live births for rhesus females is shifted to earlier gestational ages while the distribution of live births for rhesus males is shifted to later gestational ages resulting in a mean gestational age at term that is shorter for females than males [Shaughnessy et al., in press]. While a male excess occurs after 150 days’ gestation in utero, a male excess in live births occurs after 160 days (fig. 1). Hence there may be some selective pressure for female delivery earlier in gestation which is responsible for reversing the sex ratio in utero and at delivery at later gestational ages. Depending on the methodology, conflicting results have been obtained for the human fetal sex ratio. Numerous human studies [Matsunaga et al., 1963; Mikamo, 1969a, b; Pulle and Rigano, 1960; Stevenson, 1966; Szontagh et al., 1961] utilizing the nuclear sex chromatin method, have shown a male pre­ dominance both in utero (therapeutic abortions and ectopic pregnancies) and in products of spontaneous fetal death. The study of Moore and Hyrniuk [1960] was the only sizable one in which a female excess was observed in spontaneous abortions. Histologic examination of the gonads from 1,500 first trimester induced abortions revealed a male excess [Lee and Takano, 1970]. However, chromosomal analysis has demonstrated a female preponderance, both in utero

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Fig. 1. Sex ratio of rhesus fetuses in utero and at delivery.

Rhesus Fetal Sex Ratio


(therapeutic abortions) [Geneva Conference, 1966; Makino et al., 1962; Makino, 1967, 1968; Sasaki et al., 1967; Yasuda et al., 1967] and in spontaneous fetal deaths [Carr, 1963, 1965, 1967, 1971; Geneva Conference, 1966; Kerr and Rashad, 1966; Singh and Carr, 1967; Wingate, 1966], The study by Dhadial et al. [1970] was the only sizable one in which a male predominance was observed in spontaneous abortions. Since karyotypic analysis should provide more ac­ curate information as to sex, there may be a consistent inherent error in the sex chromatin method which overestimates the proportion of males. A comparison of methods within the same study population should provide reasons for this inconsistency. Wingate [1966] found complete agreement between anatomical study, nuclear sexing and chromosomal analysis in his report of a female excess in second trimester spontaneous abortions. The ratio of maletfemale human births is 1.03-1.06 indicating a male excess although the distribution of live births by gestational age is not appreciably different between males and females [Hoffman et al., 1974].

Carr, D.H.: Chromosome studies in abortuses and stillborn infants. Lancet ii: 603-606 (1963). Carr, D.H.: Chromosome studies in spontaneous abortions. Obstet. Gynec., N.Y. 26: 308-326 (1965). Carr, D.H.: Cytogenetics of abortions; in Comparative aspects of reproductive failure, pp. 96-117 (Springer, New York 1967). Carr, D.H.: Chromosomes and abortion; in Advances in human genetics, pp. 243-245 (Plenum Press, New York 1971). Dhadial, R.K.; Machin, A.M., and Tait, S.M.: Chromosomal anomalies in spontaneously aborted human fetuses. Lancet ii: 20-21 (1970). DiGiacomo, R.F.; Shaughnessy, P.W., and Tomlin, S.L.: Fetal-placental weight relationships in the rhesus (Macaca mulatto). Biol. Reprod. 17: 749-753 (1978). Geneva Conference: Standardization of procedures for chromosome studies in abortion. Bull. Wld Hlth Org. 34: 765-782 (1966). Hoffman, H.J.; Stark, C.R.; Lundin, F.E., jr., and Ashbrook, J.D.: Analysis of birth weight, gestational age, and fetal viability, U.S. births, 1968. Obstet. Gynec. Surv. 29: 651-681 (1974). Kerr, M. and Rashad, M.N.: Chromosome studies on spontaneous abortions. Am. J. Obstet. Gynec. 94: 322-339(1966). Lee, S. and Takano, K.: Sex ratio in human embryos obtained from induced abortion. Histological examination of the gonad in 1,452 cases. Am. J. Obstet. Gynec. 108: 1294-1296 (1970).

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R.F. DiGiacomo, Department of Epidemiology, SC-36, School of Public Health, University of Washington, Seattle, WA 98195 (USA)

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Makino, S.: Chromosome studies in human subjects. A summary. Jap. J. Hum. Genet. 12: 83-93 (1967). Makino, S.: Chromosome data and sex ratio in induced abortion. Mammalian Chrom. Newslett. 9: 93-94 (1968). Makino, S.; Kikuchi, Y.; Sasaki, M.S.; Sasaki, M., and Yoshida, M.: A further survey of the chromosomes in the Japanese. Chromosoma 13: 148-162 (1962). Matsunaga, E.; Tonomura, A.; Inui, N., and Honda, T.: Embryonal sex ratio in Japanese determined by the sex-chromatin test. A preliminary report. Jap. J. Hum. Genet. 8: 89 (1963). Mikamo, K.: Female preponderance in the sex ratio during early intrauterine development. A sex chromatin study. Jap. J. Hum. Genet. 13: 272-277 (1969a). Mikamo, K.: Prenatal sex ratio in man. Observations contradictory to the prevailing concept. Obstet. Gynec., N.Y. 34: 710-716 (1969b). Moore, K.L. and Hyrniuk, W.: Sex diagnosis of early human abortions by the chromatin method. Anat. Rec. 136: 247 (1960). Park, W.W.: The occurrence of sex chromatin in early human and macaque embryos. J. Anat. 91: 369 -373 (1957). Pullè, C. and Rigano, A.: Sex chromatin in the human placenta. Minerva ginecol. 12: 934-941 (1960). Sasaki, M.; Makino, S.; Muramoto, J.I.; Ikeuchi, T., and Shimba, H.: A chromosome survey of induced abortuses in a Japanese population. Chromosoma 20: 267-283 (1967). Shaughnessy, P.W.; DiGiacomo, R.F.; Martin, D.P., and Valerio, D.A.: Prematurity and perinatal mortality in the rhesus (Macaca mulatto). Relationship to birth weight and gestational age. Biol. Neonate (in press). Singh, R.P. and Carr, D.H.: Anatomic findings in human abortions of known chromosomal constitution. Obstet. Gynec., N.Y. 29: 806-818 (1967). Stevenson, A.C.: Sex chromatin and the sex ratio in man; in The sex chromatin, pp. 263-276 (Saunders, Philadelphia 1966). Szontâgh, F.E.; Jakobovits, A., and Méhes, C.: Primary embryonal sex ratio in normal pregnancies determined by nuclear chromatin. Nature, Lond. 192: 476 (1961). Valerio, D.A.; Miller, R.L.; Innés, J.R.M.; Courtney, K.D.; Pallotta, A.J., and Guttmacher, R.M.: Macaca mulatto: Management of a laboratory breeding colony (Academic Press, New York 1969). Wingate, M.B.: The sex ratio of mid-trimester abortions. J. Obstet. Gynaec. Br. Commonw. 73: 296-298 (1966). Yasuda, M.; Matsuda, N., and Fonomura, A.: Chromosome studies in human embryos by a direct method. Proc. Cong. Anom. Res. Ass. Jap. 7: 51 (1967).

Fetal sex ratio in the rhesus (Macaca mulatta).

Folia primatol. 31: 246-250 (1979) Fetal Sex Ratio in the Rhesus (Macaco mulatto) R.F. DiGiacomo and P. W. Shaughnessy1 Department of Epidemiology, S...
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