Further concepts on regulators of the sex ratio in human offspring: Interpregnancy intervals, high maternal age and seasonal effects on the human sex ratio

Abstract

Seasonally of general births and secondary sex ratio in the 17th and 18th century Canadian population. Triennial Rep., 6,[8][9][10][11][12][13][14][15]. Pallast, E.G.M., Jongbloet, P.H., Straatman, H.M. and Zielhuis, G.A. (1994) Excess seasonality of births among patients widi schizophrenia and seasonal ovopathy. Schizophr. Bull., 20, 269-276. Philpot, M., Rottenstein, M., Bums, A. and Der, G. (1989) Zielhuis, G.A. and Jongbloet, P.H. (1996) Seasonal variation in the time to pregnancy; avoiding bias by using the date of onset. Epidemiology, in press. Troya, M., Bomsel-Helmreich, O., Bertrand, P., Papiemik, E. and Spira, A. (1985) Relationship between the characteristics of the menstrual cycle and congenital malformations in the human. Early Hum. Develop., 11, 307-315. Weinberg, C.R., Baird, D.D. and Wilcox, AJ. (1995) The sex of the baby may be related to the length of the follicular phase in the conception cycle.

Sex ratio is remarkably constant

“…It has been long hypothesized that the sex of mammalian offspring (including human) is not totally dependent on a random X or Y spermatozoon, but is causally related to other factors. The length of the follicular phase of the cycle, the day of intercourse in relation to ovulation, and coital frequency have all been suspected to be involved in the determination of the gender of newborns (1)(2)(3)(4)(5)(6)(7)(8), both in humans and other mammals (9,10). This observation was attributed to rising maternal estrogen levels and changes in cervical mucus permeability during the follicular stage.…”

Temporal and spatial trends in the sex ratio at birth in Greece, 1960–2006: exploring potential environmental factors

“…The demographic variables examined include parents' age and marital status, family size and birth order, race and ethnicity. Overall, there is sufficient evidence that sex ratio at birth decreases with mother's age (Juntunen et al 1997;James 1996;Mathews and Hamilton 2005;Smith and von Behren 2005) as well as father's age, while the father-mother age gap does not significantly contribute to sex ratio determination (Gutierrez-Adan et al 2000). The partnership-status hypothesis has been cited as an additional explanatory factor.…”

“…Prior literature suggests that sex ratio varies significantly with the mother's age (James 1996 Meanwhile ''westernisation of demographic behaviour'' is a term widely used to describe decreased marriage rates, later maternity, higher shares of out-of-marriage births as well as smaller families (see among others van Nort 1960). All these trends have been shown to exert an influence on sex ratio at birth.…”

Endocrine disruptors and congenital anomalies

“…Numerous factors have been associated with the gender of the newborn, as for instance, maternal age (Juntunen et al, 1997); gonadotrophin levels ( James, 1980); interpregnancy intervals and seasonal effects ( James, 1996); maternal diabetes (Rjasanowski et al, 1998); family size and birth order (Biggar et al, 1999); length of follicular phase and moment of conception (Jongbloet et al, 1996). Other factors have been associated with a reduction of male offspring, as older age of the fathers, in vitro fertilization, non Hodgkin lymphoma, hepatitis and use of fertility drugs, such as clomiphene ( James, 1995).…”