In contrast to current general experience, autopsy percentages remain high from most neonatal intensive care units, overcoming potential selection bias towards autopsies (Biol Neonate 1969;14:286-316). Our autopsy percentage often exceeded 90% over the past 21 years, a total of 1,177 cases. A dramatic shift in our data in the prevalence of immature twins of all types (usually less than 28 weeks gestational age, usually under 800 g birth weight) may be a herald of more than just the recent increase in twinning in the United States, despite stable or slightly lower rates of low birth weight, very low birth weight, and infant death rates (Natl Vit Stat Rep 2000;48:2, 2002;50:15-7). The ten year interval, 1984-1993, had an autopsy twin prevalence of 2.97%; prevalence in the ten year interval, 1994-2003, more than doubled, to 7.29%. In 2004 through August the prevalence was 13.64%. Such findings are perhaps due to factor(s) yet to be discerned since they do not match increased twin births (up 55% since 1980 (Natl Vit Stat Rep 2002;50:19) during periods of significant improvement in neonatal care. The more recent period contains many lesions, including anomalies sometimes attributed to cocaine-related abuse and increases in intracranial hemorrhage.