Multiple Gestation Part 1: Why Twins and Higher Order Pregnancies Are so Common Today

In considering potential topics, I started thinking about multiple gestation —meaning twins and higher order pregnancy. Then, looking through posts of the past few years on The Pulse, I found a great number dealing with twins, with some unusual categories of twinning, including one that I authored on the fascinating topic of superfetation, and on some specialized situations like twin-twin transfusion syndrome and conjoined twins. But there hasn’t yet been a comprehensive discussion on multiple pregnancy overall, so let’s take a deep dive into this topic. There is a lot to cover, so this post will be Part 1 of a series.

Let’s begin with a definition and some numbers. The term multiple pregnancy, or multiple gestation, refers to any pregnancy in which there is more than one baby inside the same uterus, in contrast with a singleton pregnancy, which is the usual pregnancy, the kind that produces one baby. In humans, the overwhelming majority of pregnancies are singletons. Among natural pregnancies —those initiated without fertility treatment— the overall rate of multiple pregnancy is about 1 per 250 pregnancies. That’s overall for humanity, but there are some interesting differences when you look into geography and ethnic groups. Compared with that 1 in 250 planetwide rate, Asia and Latin American have lower rates, while central Africa has especially high rates. The central African country of Benin boasts and especially high twinning rate of 1 per 36 pregnancies! The tendency for multiple pregnancy apparently relates to genetic factors as well as to environmental factors.

As noted, the numbers cited above are for naturally occurring pregnancy, not for fertility treatment, and in cases of natural multiple gestation, the overwhelming majority are twin pregnancies. The usual cited numbers are 96-97 percent, meaning that, among natural multiple pregnancies, only 3-4 percent are higher order, meaning triplets, quadruplets, or more. The numbers change in settings of fertility treatment. In such cases, the rate of multiple pregnancy rises substantially and the percentage of higher order pregnancy increases too, especially in women who become pregnant by way of in vitro fertilization and embryo transfer (IVF-ET). There are two reasons for this. One reason is that fertility treatment involves agents that cause excessive ovulation, the release of ova (eggs). Consequently, instead of releasing just one ovum per cycle, a woman receiving such treatment produces multiple ova. By itself, this type of treatment increases the chances of multiple pregnancy, since more than one egg can be fertilized, leading to more than one blastocyst implanting into the endometrium of the uterus several days later. The other reason applies in cases of IFV-ET, which is warranted when hyperstimulation of ovulation is not enough to achieve pregnancy. In women who are to undergo IVF, multiple ova, produced in ovaries each month by way of fertility medications are extracted. Then, the ova are fertilized, in vitro (in a dish in the laboratory), with sperm from the aspiring father. The result of fertilization is a one-cell entity called a zygote. Still in the laboratory, over the course of 5-6 days, each zygote develops into an entity containing tens to hundreds of cells, called a blastocyst. Embryos that reach the blastocyst stage and that look good when examined under a microscope are selected for injection into the aspiring mother’s uterus, or into a surrogate mother. However, only a fraction of such blastocysts will successfully implant and persist into a successful pregnancy, while each cycle takes a month, it is costly, and takes a toll on the aspiring mother’s body. Consequently, the practice is to inject multiple blastocysts at once, then wait to see how many, if any, take. Ideally, this will result in one successful implantation in an optimal location, but often it results in two or more implantations. Often, several of the implantations do not progress to healthy embryos and other times, multiple embryos take root and the fertility specialist and patient decide in favor of a reduction procedure, in which one or more of the embryos are removed. Some women who undergo IVF-ET will opt for reduction down to a singleton pregnancy, even at the risk that something will go wrong, ending the pregnancy for the one embryo that looks promising at the early stage, while other women will opt for reducing the number down to two, meaning a twin pregnancy. But at the end of the day, the overall effect of IVF-ET is to increase the rate of multiple pregnancy, and, among such multiple pregnancies, also to increase the rate of triplets and other higher order pregnancies. Consequently, because of the advent of fertility treatment, twins are much more common than they were prior to the late 20th century and higher order pregnancies are much more common too.

As we continue with this multiple gestation series, we’ll discuss higher order pregnancies when appropriate, but we’ll really focus a lot on twins for two main reasons. One is that twins are the most common multiple gestation. The other is that the concepts connected with triplets and still greater numbers of gestations are really variations on the twinning concepts. Important to understand among these topics are various types of twinning, resulting first from whether twinning results from two separate zygotes (two ova, each fertilized with a sperm), or from a single zygote, and, in the latter case, resulting from the timing of when the early embryo splits into two. In Part 2, we’ll sort through these different twinning phenomena and their implications for fetal and neonatal health and also for your delivery.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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