Thomas and Kristen Hewitt – two parents in Baltimore – welcomed their identical triplets into the world, a rare case which is almost one in a million, researchers say.
The three identical boys were born on October 6 – six weeks earlier than anticipated. Dr. Jonathan Herman, an obstetrics and gynaecology doctor (OB-GYN) at Long Island Jewish Medical Center in New Hyde Park, New York, said that identical triplets are an exceptionally rare occurrence.
Hellin’s Law is the principle which says that about one in 89 pregnancies results in the birth of twins, one in 892 (squared) pregnancies, or one in 8,100 births are triplets, and one in 893 (cubed) births, or one in almost 730,000 births are quadruplets – all without the help of fertility treatments – Dr. Herman stated.
In 2003, a study – published in the Journal of Biosocial – found that identical twins and triplets are even more uncommon. One in 250 pregnancies ends up in the birth of identical twins, and only 20 to 30 per 1 million births are identical triplets.
The rate of twins, triplets, quadruplets and other multiple births have increased over the past ten years with the help of fertility treatments. According to the U.S. Department of Health and Human Services, twin birth rates have increased by 75 percent in the United States since 1980. Triplet births and other multiple births have increased at a higher rate than twin births.
The Centers for Disease Control and Prevention reported that – in 2013 – out of a total of about 3.9 million births there were more than 132,000 twin births, approximately 4,300 triplet births, about 270 quadruplet births, and 66 quintuplets births or other higher-order births (in the U.S).
When fraternal twins are conceived, the ovaries release two eggs that are fertilised by two different sperms, which is why fraternal twins do not look identical and have the same amount of genetic similarity as regular siblings (they share 50 percent of the genes).
In the case of identical twins, the woman’s ovaries only release one egg which is fertilised by one sperm. After being fertilised, the egg splits into two separate cells that share the same chromosomes. According to Dr. Herman, although the embryos may grow differently, they will still share the same DNA.
A similar process takes place in the case of identical triplets or quadruplets, when the egg – that has already been fertilised by a single sperm – splits into three or four identical cells.
There may be a genetic link as to why the egg splits into two or more cells, since identical twins usually run in the family, Dr. Herman said.
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