Triplets, the simultaneous birth of three offspring from a single pregnancy, are a rare occurrence. They happen in approximately one out of every 10,000 pregnancies without fertility treatments, contrasting with more common singleton or twin pregnancies.
The Different Ways Triplet Pregnancies Begin
Triplet pregnancies originate through distinct biological processes, leading to varying genetic similarities among the babies. These mechanisms determine whether triplets are identical, fraternal, or a combination.
Monozygotic, or identical, triplets arise from a single fertilized egg that splits into three separate embryos. The splitting can occur in stages or simultaneously. These triplets, sharing nearly identical genetic material, are always of the same sex. This type of triplet pregnancy is the least common.
Dizygotic triplets, sometimes referred to as mixed or semi-identical, occur when two separate eggs are fertilized by two different sperm. One resulting zygote then splits to form identical twins, while the other develops as a single fraternal twin. In this scenario, two triplets are genetically identical, and the third is genetically distinct, similar to a regular sibling.
Trizygotic, or fraternal, triplets are the most common type of triplet pregnancy. This occurs when three separate eggs are each fertilized by three different sperm during the same reproductive cycle. Each resulting embryo is genetically unique, similar to siblings born at different times. Trizygotic triplets can be of any gender combination and may not share a strong physical resemblance.
Factors That Increase The Likelihood of Triplet Pregnancies
Several factors can significantly increase the chances of a triplet pregnancy. These often involve the release of multiple eggs or the transfer of multiple embryos during assisted reproductive procedures.
Assisted Reproductive Technologies (ART) like in vitro fertilization (IVF) increase the incidence of multiple pregnancies. During IVF, multiple embryos may be transferred to the uterus to increase successful implantation. While modern practices favor single embryo transfer, transferring more than one embryo contributes to higher rates of multiple births.
Ovulation-stimulating drugs, such as clomiphene citrate and gonadotropins, are another factor. These medications encourage the ovaries to release multiple eggs in a single cycle. If multiple eggs are released and fertilized, the chance of conceiving multiples, including triplets, increases.
Maternal age also plays a role. Women in their late 30s to early 40s may experience hormonal changes that lead to the release of more than one egg during ovulation. This natural physiological shift contributes to a higher chance of spontaneous fraternal multiple pregnancies.
Genetic predisposition and family history can influence the chances of having fraternal multiples. A woman with a family history of fraternal twins or triplets on her maternal side may have an increased likelihood of conceiving multiples. This suggests a hereditary component linked to the natural tendency to release multiple eggs.
A woman’s parity, or the number of previous pregnancies, can slightly increase her chances of conceiving multiples again. This effect is observed in women who have had multiple previous births.