In most first-world, industrialized countries, the rate of fraternal twins has doubled over the past few decades, and in some instances, more than doubled, while the same is not true of third-world countries. A study finding this correlation between wealth and fraternal twin rates was published in the academic journal Population and Development Review, according to The Atlantic.
To illustrate this trend, the researchers of the study took a look at fraternal twin rates in certain countries in 1975 and compared them to recent data. In the United States, there were 9.5 twin births per 1,000 deliveries in 1975. In 2011, there were 16.9 twins per 1,000 births. Other developed countries showed similar reproductive trends – England and Wales showed a near-mirror increased in that time to the United States (from 9.9 to 16.1). France (9.3 to 17.4), and Germany (9.2 to 17.2) had an even bigger climb. The highest increases of all were in Denmark (9.6 to 21.2) and South Korea (5 to 14.6). Denmark has been consistently shown to have some of the healthiest individuals and most efficient health care system, according to multiple factors and indicators of health.
The rate of identical twins, or those that come from a single fertilized zygote that split into two babies very early after conception, does not seem to rise based on the wealth of a country and has stayed fairly consistent over the past decades.
The most obvious and likely factor for the increase in fraternal twins is likely due to the fact that first world countries have assisted reproductive technologies (ART) which includes ovulatory stimulations drugs, to more invasive procedures such as in vitro fertilization. The rates of the fraternal multiples is higher because physicians usually transfer more than one fertilized embryo in the hopes that at least one will burrow into the endometrial lining successfully. Often, more than one embryo completes this process, resulting in a higher multiple birth rate. ART and IVF are expensive treatments and generally not available in third world countries which explains the discrepancy.
While good nutrition can play a factor in successful pregnancies, women in richer countries tend to have higher rates of obesity, which can impair fertility and complicate pregnancies, resulting in fetal loss, but still have a much higher fraternal twinning rate than first-world countries.
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