The public perception that twin births are increasing is statistically accurate, reflecting a dramatic shift in global birth trends over the last few decades. The rate of twin deliveries has risen substantially across many regions of the world since the 1980s. This rise is directly linked to major changes in reproductive behavior and medical technology. This reveals that the twin boom is primarily concentrated in one specific type of twin and is a direct consequence of modern reproductive choices and advancements.
The Current Trend in Twin Births
Worldwide, the rate of twin deliveries increased by one-third between the early 1980s and the early 2010s, rising from nine to 12 twin deliveries per 1,000 deliveries. This absolute increase resulted in approximately 1.6 million twins being born annually in the 2010s.
In the United States, the increase was even more pronounced, with the twin birth rate rising by 76% between 1980 and 2009. This meant that in 1980, twins accounted for one in every 53 births, but by 2009, this ratio had changed to one in every 30 births. North America saw the highest increase among continents, with a 71% rise in the twin birth rate over three decades, while Europe followed with a 58% increase.
Distinguishing Between Twin Types
Twins are categorized into two fundamental types based on their biological origin: monozygotic and dizygotic. Monozygotic twins, often referred to as identical twins, result from a single fertilized egg—a zygote—that splits into two embryos early in development. Because they originate from the same egg and sperm, these twins share nearly 100% of their genetic material and are always the same sex.
Dizygotic twins, or fraternal twins, form when two separate eggs are released during the same cycle and are fertilized by two different sperm cells. These twins are genetically similar to any other siblings, sharing about 50% of their DNA, and they may be the same or different sexes. The rate of monozygotic twinning has remained relatively constant worldwide, occurring at a natural rate of about three to four per 1,000 births. Therefore, the vast majority of the statistical increase observed globally is due to a surge in the rate of dizygotic twins.
Factors Driving the Rising Rates
The increase in dizygotic twins is primarily driven by two interrelated societal and medical changes: delayed childbearing and the widespread use of assisted reproductive technologies (ART). These factors have significantly altered the likelihood of a double ovulation, which is the biological precursor for fraternal twins.
Delayed Childbearing
Women are increasingly choosing to have children later in life, and this advanced maternal age naturally raises the chance of a spontaneous dizygotic twin pregnancy. The rate of dizygotic twinning rises steadily with a woman’s age, peaking between 35 and 39 years. This biological effect is thought to be related to higher levels of the hormone FSH (follicle-stimulating hormone) in older women, which can occasionally cause the ovaries to release more than one egg in a single cycle.
The shift in the age distribution of mothers accounts for a portion of the overall rise in twin births. The twinning rate has increased most dramatically among older mothers, rising by more than 200% for women aged 40 and over between 1980 and 2009 in the United States.
Assisted Reproductive Technology (ART)
Assisted Reproductive Technology (ART), which includes procedures like in vitro fertilization (IVF) and the use of ovulation-stimulating medications, is the other major contributor to the rise in twins. Fertility drugs, such as those used for ovarian stimulation, work by encouraging the ovaries to produce multiple mature eggs in one cycle, which significantly increases the chance of multiple fertilizations.
In the case of IVF, the traditional practice of transferring multiple embryos into the uterus to maximize the chance of a successful pregnancy directly leads to a higher rate of fraternal twins if more than one embryo implants. It is estimated that fertility treatments are responsible for up to one-third of all twin births in the United States. While the number of embryos transferred in IVF is being reduced in many regions to mitigate the health risks associated with multiple births, the cumulative effect of these technologies remains a powerful influence on global twinning rates.