Higher Rates of DZ Twinning in a Twenty-First Century Birth Cohort

Area of Science:

• Reproductive biology
• Population genetics
• Biostatistics

Background:

• The Colorado Twin Registry has tracked twin births since 1984.
• Historically, Caucasian populations were expected to have a 1:1:1 ratio of monozygotic (MZ), same-sex dizygotic (DZ), and opposite-sex DZ twins.
• Previous research suggested a falling dizygotic (DZ) twinning rate, not a rising monozygotic (MZ) rate, to explain twin study enrollment biases.

Purpose of the Study:

• To evaluate recent trends in the proportion of monozygotic (MZ) and dizygotic (DZ) twins in the Colorado population.
• To investigate potential contributing factors to observed shifts in twin birth rates.

Main Methods:

• Analysis of Colorado birth records from 2006 to 2008, including 3217 twin births with identified sex information.
• Application of Weinberg's rule to estimate the monozygotic (MZ) twin rate, assuming equal birth rates for same-sex and opposite-sex dizygotic (DZ) pairs.
• Exploration of demographic factors such as parental ethnicity and maternal age.

Main Results:

• The estimated monozygotic (MZ) twin rate in Colorado from 2006-2008 was 22%, a significant deviation from historical 1:1:1 expectations.
• Corroborating reports from other states indicate similar declining MZ twin rate estimates.
• Potential contributing factors include an increasing proportion of non-Caucasian parents and an increase in average maternal age.

Conclusions:

• Recent twin birth data suggests a declining monozygotic (MZ) twinning rate, contrary to historical assumptions.
• Demographic changes, including parental ethnicity and maternal age, may be influencing twin birth proportions.
• Further research is needed to fully understand the causes and implications of these shifting twin rate trends.