The practice of in vitro fertilization has changed tremendously since the birth of the first in vitro fertilization infant in 1978. With the success of early in vitro fertilization programs in the United States, there was a substantial rise in twin births nationwide. In the mid-1990s, more than 30% of in vitro fertilization cycles resulted in twin or higher-order multifetal pregnancies. Since that time, we not only have witnessed improvements in laboratory and treatment efficacy but also have seen a dramatic impact on pregnancy outcomes, specifically regarding twin pregnancies. Because the field evolved and the risks of multifetal pregnancies became more salient, in 2019, the rate of twin pregnancies had dropped to <7% of cycles. This improvement was largely because of technical advancements and revised professional guidance: culturing embryos longer before transfer, improved freezing technology, embryo preimplantation genetic testing, and revised professional guidance regarding the number of embryos to transfer. These developments have led to single-embryo transfer becoming the standard of care in most scenarios. We used national in vitro fertilization surveillance data of all autologous in vitro fertilization cycles from 1996 to 2019 to illustrate trends in the following improved outcomes: autologous embryo transfer cycles involving blastocyst-stage embryos, vitrified embryos, preimplantation genetic testing cycles, total number of embryos being transferred per cycle, and single-embryo transfer usage over time. Among deliveries from autologous embryo transfers, we highlighted trends in singleton births over time and proportion of deliveries involving twins, triplets, quadruplets, or greater. The notable progress in reducing the rate of multifetal pregnancies with in vitro fertilization was largely attributed to a series of technical and clinical actions, culminating in an 80% reduction in the incidence of multiple births without a loss in overall treatment effectiveness.
Introduction
The practice of in vitro fertilization (IVF) has changed tremendously since the birth of the first IVF baby in 1978. We have witnessed significant progress in the field of assisted reproductive technology (ART) with the advent of novel technologies and through the refinement of practice patterns based on evolving evidence.
The problem of multifetal pregnancy has been known since the inception of ART. With the success of early IVF programs in the United States, there began a concurrent rise in twin births nationwide. Iatrogenic twin birth increased 66% between 1980 (pre-IVF) and 2003; iatrogenic triplet births increased by approximately 500% during that same time interval. A study by Kulkarni et al estimated that by 2011, a total of 36% of twin births and 77% of triplet and higher-order births in the United States resulted from infertility treatment. Although only some of these increases were because of IVF (and the remainder because of ovulation induction without egg retrieval), the problem was real and significant.
Twin pregnancies and pregnancies with >2 fetuses (higher order) carry significant maternal, fetal, and perinatal risks. The rates of essentially every obstetrical complication are more common in twin pregnancies and worsen with increasing plurality. For the pregnant patient, pregnancy-related medical morbidities are more common in twin pregnancies than in singleton pregnancies. Twin pregnancies have higher rates of hyperemesis gravidarum, gestational diabetes mellitus, hypertensive disorders, anemia, hemorrhage, cesarean delivery, and postpartum depression than singleton pregnancies. Moreover, fetal risks in twin pregnancies are significant. Compared with singleton pregnancies, twin pregnancies pose a 5-fold increased risk of stillbirth, a 7-fold increased risk of neonatal death, and a 6-fold increased risk of preterm birth, with significantly higher rates in pregnancies with >2 fetuses. , Most neonatal risks in twin pregnancies result from high rates of preterm birth. These risks include retinopathy, bronchopulmonary dysplasia, hypoglycemia, and necrotizing enterocolitis. Moreover, prematurity poses a significant risk of high-grade interventricular hemorrhage, which is a leading cause of the subsequent development of cerebral palsy. When outcomes of twin pregnancies were analyzed, the incidence of cerebral palsy in twin pregnancies was 7 per 1000 live births, whereas the incidence of cerebral palsy among triplet pregnancies was 28 per 1000 live births, compared with an overall prevalence of cerebral palsy in the United States being 2.6 to 2.9 per 1000 live births.
Within the IVF context, it has long been realized that the chances of multifetal pregnancies increase with the transfer of greater numbers of embryos. The solution to reducing multifetal pregnancy while maintaining a high incidence of pregnancy remained elusive, because decreasing the number of embryos transferred would significantly diminish the prospects for a successful birth. National professional guidance and technical advances were critically needed.
When the field of assisted reproduction was in its infancy, there was a void in clinical guidelines aimed at reducing the twin risk with infertility treatment. After the Society for Assisted Reproductive Technology (SART) was founded, the first national IVF report was published in 1988. A decade later (1997), the Centers for Disease Control and Prevention (CDC) began reporting on IVF outcomes as well, and the 2 organizations have since worked collaboratively to improve clinical practice (ie, advocating for fewer embryos transferred), expand insurance coverage (including state-mandated infertility treatment coverage), and promote scientific publications to optimize patient and fetal outcomes. Both organizations have transitioned clinic-level summary reporting of “fresh” and “frozen” IVF cycles to cumulative live-birth rates (births stemming from 1 attempt at egg retrieval) and have highlighted singleton births to maximize the focus on optimal perinatal outcomes rather than just a positive pregnancy test or “pregnancy rate” that included multiple pregnancies.
Dealing with the rate of multifetal pregnancy in IVF has been a 25-year endeavor. In the early days of IVF treatment, >1 embryo was typically transferred per attempt to compensate for the low implantation rate with single-embryo transfers (SETs). This continued through the mid-1990s, where more than 30% of IVF cycles resulted in twin or higher-order multifetal pregnancies (and more than 50% of the children born from IVF were of these pregnancies) ; in 2019, the rate had dropped to <7% of IVF cycles (preliminary national data). The dramatic reduction was largely because of a series of actions leading to the implementation of SET as the standard of care for most treatment cycles. For instance, in women aged <35 years, the national average for IVF cycles resulting in singleton births has increased from 14% (1996) per intended egg retrieval to 36% per intended egg retrieval (preliminary national data of 2019).
The tremendous progress in reducing the rate of multifetal pregnancies in IVF was largely because of technical advancements and revised professional guidance reviewed herein: culturing embryos longer before transfer, improved freezing technology, preimplantation genetic testing (PGT) of embryos, and comprehensive professional guidance regarding the number of embryos to transfer.
Technical Improvement #1: Extended Embryo Culture
The ability to culture embryos to the blastocyst stage has allowed for the reduction of multiple embryos being transferred per attempt while improving pregnancy outcomes. When IVF technology was in its infancy, laboratories used media that supported the growth of embryos for only 2 or 3 days (until the embryo reached the four- to eight-cell stage, or the “cleavage stage”). With time, scientists and embryologists developed a better physiological understanding of mammalian embryo energy requirements, which allowed for optimization of culture medium (ie, reduced reliance on glucose as an energy substrate, specific oxygen requirements, chemical composition of culture media, and type of protein supplement). Basic science research on the laboratory embryo culture environment has since evolved to focus on extending embryo development to 5 or 6 days (until the embryo has reached the 100- to 200-cell stage, or the “blastocyst stage”). Initially, improvements in blastocyst formation were achieved using 2-stage media (simple medium for the first 3 days, followed by a more complicated medium, including essential amino acids and switched energy substrates). These media were designed to replicate the fallopian tubal and intrauterine environments. Later, the single-step medium achieved similar blastocyst formation rates as the 2-stage media but provided the advantage of not needing to handle (and disturb) the growing embryos on day 3 of embryo development. This allowed for effective embryo culturing to the blastocyst stage. Blastocysts are much more likely to implant than earlier-stage embryos, in part, because some earlier-stage embryos experience growth arrest in culture and do not develop into blastocysts (allowing for better embryo selection for transfer) and because of better temporal synchronization between the embryo and uterine endometrium. Consequently, as blastocyst-stage embryos are more likely to establish pregnancy than cleavage-stage embryos, fewer embryos need to be transferred to achieve similar pregnancy rates. Thus, “extended” culture permits a reduction of the number of embryos being transferred without affecting overall pregnancy rates. This trend toward increased use of blastocyst embryo transfers is shown in Figure 1 , A.
Technical Improvement #2: Embryo Vitrification for Cryopreservation
Advancements in embryo freezing, or cryopreservation, indirectly permitted an increase in SET. There are 2 methods to cryopreserve human embryos: “slow freezing” and “vitrification.” Slow freezing (or equilibrium freezing) and subsequent frozen embryo transfer were first described in 1983. Slow freezing was initially applied to cleavage-stage embryos and resulted in an embryo thaw survival rate of approximately 80% to 90%. , Slow freezing allowed embryos to be cooled in equilibrium with cryoprotective agents to minimize the impact of external osmotic stress during freezing. However, a known limitation of this technique was the high rate of embryo loss and/or embryo damage because of the formation of harmful extracellular ice crystals during the freezing process. , Initially, slow freezing was the only option for cryopreservation. Eventually, this technique was applied to blastocyst-stage embryos and resulted in an embryo thaw survival rate of approximately 70% to 90%, but with implantation rates failing to equal those achieved following fresh embryo transfer. Because outcomes with frozen embryo transfer were suboptimal, fresh transfers of multiple embryos were the norm to maximize IVF outcomes and minimize the harm of cryopreservation or thawing attrition; however, this came at the expense of multifetal pregnancies.
Conversely, vitrification (or nonequilibrium freezing) was later described in mouse embryos in 1985 ; however, this technique was not used in humans until 2008 and was adopted because of a significantly improved efficiency in the freeze-thaw process. Compared with the slow freezing technique, vitrification uses a higher initial concentration of cryoprotectant and a more rapid cooling rate and effectively eliminates extracellular ice crystal formation. The vitrified solution does not transition from a liquid to a solid state, which allows for maintenance of the molecular distribution and physiological processes of the embryo and better embryo survival outcomes. , Compared with slow freezing, vitrification allowed for gamete and embryo cryopreservation with minimal, if any, damage and excellent thaw success (well above 90%). Moreover, the concomitant use of PGT in many cases, which normally requires short-term embryo freezing while awaiting the results of the genetic testing, has increased the use of blastocyst cryopreservation via vitrification. We have witnessed a marked increase in the trend toward frozen embryo transfers, which started to rise in 2010 for all age groups after increasing acceptance of this new freezing technology ( Figure 1 , A). We can attribute some of this success to chromosome screening of blastocyst-stage embryos and subsequent embryo transfer. Accordingly, the percentage of live-born deliveries after frozen embryo transfers has increased since 2010.
Technical Improvement #3: Preimplantation Genetic Testing for Aneuploidy
In addition, increasing the use of PGT for aneuploidy (PGT-A) has contributed to the rise in singleton pregnancies after IVF. The first report of genetic testing on human embryos came out of the United Kingdom in 1990, followed by the first cohort of successful pregnancies that resulted after aneuploidy screening in 1995. Since that time, aneuploidy testing began to be used to select genetically normal, or euploid, embryos for transfer and became referred to as preimplantation genetic screening (PGS). PGS was initially intended to optimize IVF outcomes for patients with recurrent pregnancy loss or advanced maternal age. Initial testing for a limited number of chromosomes was performed using either polar body screening or assessment of 1 or 2 cells in cleavage-stage embryos. Since then, more comprehensive array and sequencing approaches that are capable of screening all 24 chromosomes have become the dominant approach. When biopsy is performed on a cleavage-stage embryo, this technique may compromise subsequent embryo development. The use of trophectoderm biopsy on a blastocyst-stage embryo, which seems to cause little or no harm to the embryo, has become predominant. We have witnessed increasing use of PGS (now referred to as PGT-A), and several different molecular techniques have been developed for genetic sequencing of 24 chromosome copy numbers. The implementation of PGT-A for the detection of euploid embryos now allows for the selection of a single embryo for transfer that has the chance of implantation and delivery exceeding 50%, largely independent of age. The use of PGT-A as a universal screening technique in all IVF patients has yet to be accepted, particularly as recent publications indicate that PGT-A may not provide a significant improvement in the live-birth rate in patients with a favorable prognosis. , In addition, PGT-A has important limitations for consideration, including cost-effectiveness, potential harm of biopsy, accuracy of test results (including mosaicism and false-positive and false-negative results), and impact on total reproductive potential per IVF cycle considering testing inaccuracy. Despite the ongoing debate about the value of PGT-A and its impact on live-birth outcomes, the use of PGT-A has played a critical role in the success of SET outcomes. PGT-A has not only been crucial to the success of SET but also played an important role in the acceptance of SET by both patients and physicians. There has been a rapid rise in the use of PGT-A. Figure 1 , A, shows the percentage of cycles using PGT compared with all IVF cycles performed nationally. The number of IVF cycles using PGT has gradually increased since 2014 and encompassed approximately 42.7% of all cycles in 2019 (preliminary national data). Figure 1 , B, shows the percentage of age-stratified embryo transfers in which embryos underwent PGT, where we can see that the uptake of PGT is most prevalent in patients aged 38 to 40 years. When characterizing all US IVF cycles in 2019, PGT was mostly used on patients aged <35 years (14.3% of all cycles), followed by 35 to 37 years (10.6%), 38 to 40 years (10.5%), 41 to 42 years (4.8%), and ≥43 years (2.4%).
Updated Professional Guidance: Increased Focus on Infant Health
The psychological shift in embracing SET among patients and providers was a critical piece to the pregnancy plurality puzzle. This change was largely because of an increasing awareness of the significant risks of twin pregnancies and higher-order pregnancies among healthcare providers. Concurrently, there was a shift in patient attitude toward increased confidence that a single embryo could provide a reasonable chance of live birth.
Studies evaluating the efficacy of SET in young women found similar pregnancy rates and live-birth rates compared with multiple-embryo transfers, with a substantial decrease in multiple pregnancies. Evidence from these well-controlled randomized controlled trials led to confidence on behalf of the American Society for Reproductive Medicine (ASRM) and SART that SET was acceptable and appropriate. The CDC began to promote SET through its “Having Healthy Babies One At a Time” campaign starting in 2012. In 2017, to reduce the rate of twin pregnancy and encourage SET, the ASRM published restrictive guidance on the number of embryos to transfer, most notably advocating for SET for women aged <38 years. Before this (dating back to 2008), the ASRM had begun proposing limits on the number of embryos to transfer; however, at that time, the goal was a reduction in the rate of triplet (not twin) pregnancy; this was accompanied by a remarkable decline in triplets, which was instrumental in the eventual acceptance of SET. The advocacy from ASRM, RESOLVE: The National Infertility Association, and other professional organizations has played an important role in increasing the prevalence of insurance coverage for IVF, which has encouraged fewer embryos per transfer and policies favoring the use of SET. Accordingly, the use of SET is more likely in instances where patients have IVF insurance coverage, likely driven by the reduction of morbidity and healthcare costs associated with multifetal pregnancies. Moreover, advocacy policies have impacted national SET trends through state-mandated IVF insurance coverage, which has become a vehicle for improving SET usage. The average number of embryos per embryo transfer is significantly lower in states with IVF mandates, and states without mandates for IVF coverage have higher rates of multiple births after IVF. ,
With these advancements in laboratory techniques, improvements in embryo culturing, embryo vitrification success, and increased availability of PGT, we have witnessed a reduction in the numbers of embryos being transferred ( Figure 2 , A) and an expansion of SET, nationally . Since 2012, there has been a rise in SET among all age groups pursuing IVF in the United States ( Figure 2 , B). This has been made possible even in older patients because of the advent of PGT-A. In the age group of <35 years, SET increased from 0.1% in 2003 to 82.4% in 2019 (preliminary national data), and in the age group of 38 to 40 years, SET increased from 0.2% to 73.3% during the same period. Accordingly, there has been a remarkable decline in the proportion of IVF children born as twins, triplets, quadruplet, or greater ( Figure 3 , A), and the decline in the proportion of births involving twins, triplets, or greater is even more dramatic ( Figure 3 , B).
