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Using Taiwanese administrative records, we compile the data of all single and multifetal births, and find the following surprising facts in recent years. A) Among all twins, there is a positive relationship between family wealth and the proportion of 1-boy-1-girl twins (with corresponding declines in the proportion of 2-boy or 2-girl twins). B) Conditional upon the fact that the parents already have a singleton first birth, the hazard rate that they realize a second birth is higher if parents are wealthier or if their first birth is a girl. C) Conditional upon the fact that the parents already have a singleton first birth, the probability that they accomplish a mixed-gender sibship after their second birth is higher if they are wealthier, or if their first birth is a girl. We hypothesize that it is the IVF (in vitro fertilization) that is behind these observations. Since IVF is more affordable to wealthier families, parental intrinsic preferences are more likely to be realized for wealthier groups. We test this hypothesis empirically. We argue that A), B) and C) are all consistent with Taiwanese traditional son preferences, despite that A) and C) appear to be about sibship composition.