Equal­ity be­tween the sexes achieved— but only dur­ing mo­ment of con­cep­tion

The China Post - - LIFE - BY MAL­COLM RIT­TER

Ev­ery year, slightly more boy ba­bies than girl ba­bies are born world­wide. But back when sperm meets egg, the two sexes are con­ceived in equal num­bers, a new study sug­gests.

That con­tra­dicts the idea found in many text­books and sci­en­tific ar­ti­cles that males are in the ma­jor­ity at con­cep­tion, re­searchers said.

And it im­plies more fe­males than males die be­fore birth, re­sult­ing in the ex­cess of male births, says Steven Orzack, a study au­thor.

“We don’t have good in­for­ma­tion on the cause of this dif­fer­ence,” he said.

The work, re­leased Mon­day by the Pro­ceed­ings of the Na­tional Academy of Sciences, also es­ti­mates the ra­tio of males to fe­males at var­i­ous points in preg­nancy. It finds a see- saw­ing pat­tern over that time in which sex is more prone to die in the womb, as var­i­ous ge­netic in­flu­ences take their toll.

“We’re gain­ing fun­da­men­tal new in­sights in the bi­ol­ogy of hu­mans in the first nine months of life,” Orzack said.

That bi­ol­ogy “un­folds dif­fer­ently, start­ing soon af­ter con­cep­tion, be­tween boys and girls,” he said.

Orzack is pres­i­dent of the non­profit Fresh Pond Re­search In­sti­tute in Cam­bridge, Mas­sachusetts. He re­ports the work with sci­en­tists from Har­vard and Ox­ford uni­ver­si­ties and else­where.

In gen­eral, around 105 boys are born for ev­ery 100 girls world­wide. The im­bal­ance, first no­ticed cen­turies ago, is a nat­u­ral phe­nom­e­non not due to se­lec­tive abor­tion of girls in some ar­eas.

The study au­thors drew on a va­ri­ety of sources for in­for­ma­tion on the male­fe­male ra­tio through­out preg­nancy, in­clud­ing abor­tions, ge­netic sampling of fe­tuses in the womb, and fe­tal deaths.

To es­ti­mate the sex ra­tio at con­cep­tion, they ex­am­ined data on nearly 140,000 em­bryos that had been rou­tinely screened at fer­til­ity clin­ics in the United States and else­where for ge­netic prob­lems.

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