The Science Behind Natural Pregnancy With Multiples

Pregnancy with multiples is a growing occurrence in the United States, partly due to the increased use of fertility treatments with ovulation-inducing drugs as well as what’s known as assisted reproductive technology (ART). Not all cases of twinning are caused by fertility treatments, however. Although it is rare, a woman can become pregnant naturally with quadruplets! Even though pregnancies with multiples are inherently more risky for both mother and her babies, as medical knowledge continues to advance, more and more women carrying twins, triplets, and even quadruplets can have successful outcomes. This article discusses some of the different types of twinning that can occur naturally.

Monozygotic twins (known as identical twins) result from the division of a single fertilized egg; this happens for unknown reasons and is completely random. The rate of this type of “twinning” is not influenced by heredity, the mother’s age, or any other known factors. Monozygotic twins appear approximately 2.3 to 4 times out of every 1,000 pregnancies in women of all races. Monozygotic twins are always the same sex (since they result from the fertilization of a single ovum by a single sperm). Normally, the twins share the same physical characteristics (skin, hair, eye color and body build) and even the same genetic features (blood characteristics like ABO, M, N, haptoglobin, serum group; histocompatible genes, etc.). Sometimes, these twins are mirror images of one another – with one being right-handed and the other, left-handed. Believe it or not, a process called supertwinning can happen - where a single fertilized ovum splits into four; this can result in quadruplets, or, if one is eliminated, in triplets.

Dizygotic twins, on the other hand, result from 2 ova and 2 sperm. These twins are no more similar than any siblings would be; they may be the same or different sexes and may or may not have the same blood type. 75% of dizygotic twins are the same sex (both twins are male in about 45% of cases, and both are female in about 30% of cases). Unlike monozygotic twins, the occurrence dizygotic twins are influenced by many factors. Race is a factor - for example, spontaneous dizygotic twinning occurs in 1.3 out of 1000 pregnancies in Japan; in 49 out of 1000 pregnancies in Nigeria, and in 12 out of 1000 pregnancies in the United States. Genetic factors and age are also important. It’s believed that dizygotic twinning is inherited via the female descendants of mothers of twins; the father’s genetic contribution is not thought to play any part. Women who are dizygotic twins or who are siblings of dizygotic twins have a higher twinning rate among their own children. Age is also a factor, as rates of dizygotic twinning are highest amongst women between 35 and 40 years of age. Finally, women who have taken oral contraceptives for many years and then stop them in order to become pregnant are more likely to have dizygotic twins – it’s believed that this may be the body’s way of having a rebound – where the ovaries release two eggs instead of just one.

Prenatal care during pregnancy is important for all women, but even more so for women carrying multiples. The maternal cardiovascular, respiratory, gastrointestinal, renal, and musculoskeletal systems are all subject to extra stress in multiple pregnancy. Women and their physicians must be on the lookout for conditions such as maternal anemia; urinary tract infection; preeclampsia (high blood pressure); hemorrhage either before or after delivery; and uterine atony. The support and guidance of an Ob-Gyn are extremely important to ensure a good outcome for the babies and the mother.