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 rise is almost entirely due to fraternal twins, or those babies who are conceived at the same time but come from two ova and are fertilized by two different sperm. They are no more genetically similar than any other set of siblings, and can be both male, both female, or one male and one female. Because they don’t share a placenta or amniotic sac, each having their own, complications are slightly less. It is known that mothers who are approaching peri-menopause, or the time surrounding menopause, tend to have more erratic ovulation that leads to the release of multiple ova during a single ovulatory cycle. This means older women tend to have fraternal twins more frequently. Other factors include how many children the woman has had before (the more children she has had the more likely she is to have fraternal twins.) Genetic factors can also play a part, with fraternal twins happening more frequently in some families.
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.
Some of the disparity of fraternal twinning in richer countries may be explained by the fact that women with the means to do so are having children later in life, which increases their chances of having fraternal twins. In the United States, the average age of first-time birth is 27, and the country who has the average of eldest first time mothers is Greece, with the average age being 31-years-old.
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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