Pathology of the Placenta in Singletons after Assisted Reproductive Technology Compared to Singletons after Spontaneous Conception: A Systematic Review

Abstract

Background: We reviewed the literature comparing the pathological characteristics of singleton births conceived after assisted reproductive technology (ART) with those after spontaneous conception. Methods: We reviewed PubMed, EMBASE, Ovid MEDLINE, Google Scopus, Scholar, Cochrane Central Register of Controlled Trials and the Web of Science for the previous 10 years, up to November 2022. Results: Four eligible studies included 3445 placentas, 806 after ART (IVF/ICSI). Placentas after ART differed in frequency of retroplacental and marginal hematomas (p = 0.04), increased thickness (p = 0.02), higher overall occurrences of vascular and anatomical pathology (p < 0.001) and more frequent marginal (p = 0.001) and membranous (p = 0.02) umbilical cord insertion than placentas from non-ART pregnancies. Conclusion: Further research is needed to determine the extent to which these placental changes in ART pregnancies alter its function and pregnancy outcome.

Keywords:

• Placenta
• pathology
• assisted reproductive technology
• embryo transfer
• in vitro fertilization

Introduction

The rise in “assisted reproduction technology” (ART) observed in the last years is attributed to the ongoing growth of infertility among couples in developed countries [1]. Embryos can be transferred fresh, immediately after culturing in the incubator, or frozen, which are transferred later in an upcoming cycle [2]. The growing policy of single embryo transfer (ET) due to reducing multiple pregnancy rates results in the growing proportion of frozen embryo transfer (frozen ET).

Due to unknown reasons, fresh ET pregnancies in comparison to spontaneous pregnancies result in higher preterm birth rates, lower birth weights, and maternal complications during pregnancy. Regardless of known factors for preterm birth and lower birth weight, such as higher maternal age, lower parity, and various gynecologic infertility conditions and procedures, new-borns conceived by ART have two times higher probability of preterm birth [3]. Pregnant women after fresh ET have more frequently ovarian hyperstimulation syndrome, placenta previa, vasa previa and placental abruption. Fetuses are more frequently affected by fetal growth restriction (FGR) [4,5].

These complications are not characteristic of newborns following frozen ET. Their birth weights, gestational age, and pregnancy bleeding rates are similar to pregnancies following spontaneous conception. There are fewer births following frozen ET since freezing is only done in cases of embryo surplus or ovarian hyperstimulation. They tend to be born at term, have higher birthweights, and may even be large for gestational age [6]. This is the main reason for the growing popularity of “freeze-all” embryo strategy. Pregnancies after frozen ET have a substantially higher risk of developing pregnancy-induced hypertension (PIH) and preeclampsia [7,8]. A suggested mechanism is the absence of the corpus luteum (CL), a feature of hormone replacement therapy (HRT) cycles; subsequently, the absence of hormones produced by the CL could adversely affect the maternal cardiovascular system [9].

How ART has been found to increase the risks of obstetric morbidity is still debated. Most of these adverse outcomes are related to abnormal placentation [10,11]. It is hypothesized that different aspects related to ART could negatively influence the placentation process.

The aim of this article is to evaluate and better characterize the pathological features of the placenta of singleton births conceived with ART compared to placentas after spontaneous conception, using a systematic review of published research.

Methods

Search strategy

We conducted an electronic search using PubMed, EMBASE, Ovid MEDLINE, Google Scopus, Scholar, Cochrane Central Register of Controlled Trials and the Web of Science databases. The following medical terms (Mesh), keywords and their combinations were used “placenta” AND “pathology” (Fig. 1). The search and selection criteria were limited to English language and human studies published in the last 10 years up to November 2022.

Figure 1. PRISMA flowchart summarizing the study screening and selection process.

Study selection

The screening of titles, abstracts, and full-text articles was conducted independently by the authors for determining the final eligibility criteria. Disagreements were resolved through scientific discussions; the general characteristics of the studies, including the first author’s name, article title, journal name, country of study, publication year, study design, sample size, population characteristics, and pregnancy outcomes were extracted from the studies and assessed. To prevent extraction and data entry errors, a control check between the final data used in the systematic review and the original publications was conducted by all authors.

Inclusion/exclusion criteria

Inclusion criteria included articles on placental pathology related exclusively to term singleton births conceived after fresh or frozen in vitro fertilization (IVF), including intracytoplasmic sperm injection (ICSI) cycles, compared with those after spontaneous conception. Placental pathology was categorized as anatomical, inflammatory or vascular. We excluded articles that compared placental pathological features only in the ART group. Two authors (UB and SK) independently reviewed all included articles.

Data extraction

The review was performed by PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement. Then three reviewers (HBF, BP, and MV) independently collected data on the outcomes of pathological placental findings. The extracted information included the name of the author, year of publication, type of clinical trial, final sample size, comparison group, measurement tool and results.

Assessment of risk of bias

The quality of the articles was evaluated using a Newcastle-Ottawa Scale (NOS) for assessing the risk of bias in randomized trials. Methodological quality assessment of the included studies was performed by evaluating the risk of bias in an individual study using the NOS for assessing the quality of nonrandomized studies in meta-analyses which evaluates the quality of nonrandomized studies in terms of selection, comparability, and outcomes. Studies with scores above 6 were considered as high quality, 4–6 as moderate, and those with scores below 4 as low quality [12].

Results

Literature search results

The search strategy yielded 223 potentially relevant articles. Based on the inclusion criteria, 23 articles were identified for further full-text evaluation. Finally, 4 articles were included, containing data from 3445 placentas, of which 806 were pregnancies according to ART (Fig. 1).

From NOS, the risk of bias assessment of the included studies is shown in Table 1.

Table 1. Assessment of the risk of bias using the Newcastle-Ottawa scale.

Authors	Selection	Comparability	Outcome
Joy et al. [13]	**	*	**
Yanaihara et al. [14]	***	*	***
Herman et al. [15]	**	*	***
Sacha et al. [16]	**	**	***

Legend: *High risk of bias; **Moderate risk of bias; ***Low risk of bias.

Study characteristics

We included two studies in which pathologists examined the placenta according to the consensus statement of the 2016 Amsterdam Placenta Workshop Group [13] and two studies in which other pathological features were observed. The study characteristics are listed in Table 2.

Table 2. Characteristics of the included studies.

Author, year	Study design	Sample size of participants	Duration	Evaluated groups	Amsterdam Placental Workshop	Conclusions
Joy et al. [13]	PCT	Singleton pregnancies (n = 89) divided into 3 groups: 38 women in the ART group (fresh ET), 21 conceived by IVF, and 17 by ICSI, 47 women in the control group (conceived spontaneously after at least 1 year of exposition) and 21 women in the infertility group (conception following untreated infertility > 1 year).	10 months	89 placental pathology reports (spontaneously = 39, infertility = 17, ART = 33)	No	Placentas from ART: had significantly increased thickness (p = 0.02) and greater frequency of retroplacental and marginal hematomas (p = 0.04) than placentae from spontaneously conceived pregnancies. Within the ART group: placentae from ICSI pregnancies show a significantly increased thickness (p = 0.04) when compared to those from IVF pregnancies.
Yanaihara et al., 2017^14	PCT	Singleton pregnancies (n = 1610) were divided into 2 groups: spontaneous pregnancy group - control group (n = 1453) and IVF pregnancy not including the ICSI group (n = 157).	18 months	1610 placental pathology reports (spontaneously = 1453, IVF = 157)	No	Significant difference in velamentous umbilical cord insertion (p = 0.026) in IVF pregnancy. No differences in placental weight and umbilical cord length between placentas after spontaneous and IVF pregnancies.
Herman et al. [15]	RCT	Singleton pregnancies (n = 1114) were divided into 2 groups: conceived with IVF (n = 105, 38 fresh ET, 12 frozen ET and 24 oocyte donation) and spontaneously - control group (n = 1009).	10 years	1114 placental pathology reports (spontaneously = 1009, IVF = 105)	Yes	Higher rate of VUE in IVF pregnancies (p = 0.007). No difference was demonstrated between IVF and control group in placental weight, composite of MVM and FVM lesions.
Sacha et al. [16]	RCT	Singleton pregnancies (n = 632) divided into 2 groups: spontaneous pregnancy group - control group (n = 121) IVF pregnancy (n = 511) fresh ET (n = 230) fresh ET, including ICSI group (n = 281).	13 years	632 placental pathology reports (spontaneously = 121, IVF = 511)	Yes	ART pregnancies had a more frequent anatomical pathology (p < 0.001): more placentas at the extremes of weight (small: p = 0.04; large: p = 0.04) more frequent marginal (p = 0.001) and membranous (p = 0.02) cord insertion. Spontaneous pregnancies had more frequent low-grade VUE (p = 0.025). ART pregnancies had a more frequent vascular pathology (p < 0.001), although frequencies of MVM and FVM were similar between groups: 11.4% (MVM) and 11.0% (low- and high-grade FVM) of ART pregnancies vs. 10.7% (MVM) and 9.1% (low- and high-grade FVM) of spontaneous pregnancies.

PCT: prospective cohort study, RCT: retrospective cohort study, ART: assisted reproductive technology, IVF: in vitro fertilization, ET: embryo transfer, ICSI: intracytoplasmic sperm injection, frozen ET: frozen embryo transfer, MVM: maternal vascular malperfusion, FVM: fetal vascular malperfusion, VUE: villitis of unknown etiology.

The first of the latter studies is by Joy et al. who analyzed a total of 89 placentas, 39 in the spontaneous conception group, 17 in the untreated infertility group and 33 in the ART (IVF and ICSI) group. Placentas were collected at the Royal Fertility Center, Belfast, and the Royal Jubilee Maternity Service, Belfast, from single pregnancies with a birth of ≥ 24 weeks’ gestation. The duration of the study was 10 months. They included antenatal complications such as pregnancy-related hypertension, gestational diabetes mellitus, preterm labor, intrauterine growth restriction, preterm premature rupture of membranes, low birth weight, late miscarriage (20–24 weeks gestation) and antepartum hemorrhage including placenta previa and abruptio placentae, and perinatal mortality. The study showed significantly increased thickness with a greater frequency of retroplacental or marginal hematoma after ART conception (21.2%, p = 0.04) compared with the control group (2.6%) and the infertility group (11.8%), especially in the intracytoplasmic sperm injection (ICSI) group (p = 0.04) [14]. Increased placental thickness has previously been associated with increased perinatal risk and death. Fortunately, this was not observed in this study, perhaps due to the relatively small groups. Importantly, the consistent increase in placental thickness with increasing frequency of antepartum hemorrhage and placental retroplacental or marginal hematoma that reached statistical significance was observed, even in the small numbers studied [10].

A similar study was conducted by Yanaihara et al. who examined 1610 placentas from uncomplicated singleton pregnancies with vaginal delivery at ≥ 37 weeks gestation, of which 157 were conceived after IVF. They collected placentas at Yanaihara Women’s Hospital between September 2015 and March 2017. In addition to comparing some perinatal characteristics, they examined the size of the cross-sectional area of the placenta and umbilical cord, the weight of the placenta, the length of the umbilical cord and the insertion site of the umbilical cord. The frequency of velamentous umbilical cord (20.3%, p = 0.026), which increases the risk of severe complications including fetal death, was significantly increased in IVF placentas [15].

The other two studies examined placentas according to the consensus statement of the Amsterdam Placenta Working Group [13]. Herman et al. compared 1114 singleton births with a diagnosis of FGR whose mothers had been diagnosed with chronic, gestational hypertension or preeclampsia. Of these, 105 patients were conceived after IVF (fresh and frozen ET) and 1009 were spontaneously conceived. The records of all deliveries at the Edith Wolfson Medical Center between December 2008 and December 2018, whose placentas were sent for histological examination, were reviewed. When the placenta was examined, placental weight and rates of maternal and fetal vascular malperfusion lesions were similar between groups. Villitis of unknown etiology was more common in the IVF group (16.2%, p = 0.007) [16].

Sacha et al. present a detailed assessment of pathological findings in placentas from term singleton births conceived with fresh ET compared to spontaneous pregnancies. They included 632 women, of whom 511 conceived with fresh ET (including ICSI) and 121 after spontaneous conception. All placentas were obtained from Massachusetts General Hospital. Non-ART patients were included if there was no documented history of fertility treatment in their medical record and their placenta underwent full pathologic examination for benign (not complication-related) indications. Fresh ET were conceived at their academic institution from 2004 to 2017 or conceived without ART from 2000 to 2004. In ART pregnancies, there were more placentas with extreme weight (small: p = 0.04; large: p = 0.04) and more frequent marginal (p = 0.001) and membranous (p = 0.02) cord insertion. Spontaneous pregnancies had more frequent low-grade FVM (p = 0.025). ART pregnancies had a more frequent vascular pathology (p < 0.001), although frequencies of MVM and FVM were similar between groups, observed in 11.4% (MVM) and 11.0% (low- and high-grade FVM) of ART pregnancies versus 10.7% (MVM) and 9.1% (low- and high-grade FVM) of spontaneous pregnancies [17].

Discussion

The pathological features of the placenta according to ART have only recently become of interest to researchers and therefore few studies have been conducted. Reports to date are limited, either due to small sample size, lack of experience in treating these cases, or selection bias possibly present in some studies.

ART independently associated with increased pregnancy complications, which is already well known [18–21]. In 2016, the Amsterdam Placenta Workshop Group consensus statement was introduced. They set standards for placenta examination and allowed clinical research and comparisons.

In our study we reviewed the literature for possible associations between placental pathology and pregnancy outcomes in IVF/ICSI and spontaneous pregnancies.

Until recently, unfavorable obstetric placental outcomes were mainly associated with abnormal placentation, namely abnormally invasive and low-lying placentas. Little was associated with the anatomical, vascular or pathological abnormalities of the placenta [22].

Several studies show a significant association of ART with increased anatomical and vascular pathology of the placenta, especially in the setting of lower mean birth weight, which warrants further investigation. Lower mean birth weight and a larger placenta could be an indicator of less efficient transfer of nutrients from the placenta to the fetus [23]. A large registry-based study found that the use of ART was significantly associated with larger placentas [24]. Excessive estrogen levels [25–27] and manipulation of the embryo in culture [28,29] were associated with altered trophoblast development and uterine spiral artery invasion. Alteration of uteroplacental blood flow or disruption of placental metabolism could contribute to the increased anatomical and vascular pathologies observed in the ART group.

The rate of vascular pathology was similar in the different studies in the group of spontaneous pregnancies [30]. These studies reinforce the notion that in singleton pregnancies with uncomplicated deliveries, the placenta often develops mild vascular or thrombotic lesions that have no known clinical consequences in the short term. However, an increasing number of studies report that the burden of these lesions is greater in the ART pregnancies and is associated with an increase in the frequency of preeclampsia, even though there were no significant differences in pathological findings rate such as MVM and FVM.

A prospective study at a single tertiary center found that older maternal age was associated with a higher frequency of MVM lesions in the placenta. These findings suggest an independent influence of AMA on placental function and suggest further investigation in larger samples [31].

Adequate vascularization of the placental bed is crucial because failure of physiological transformation is considered the anatomical basis for decreased blood flow to the intervillous space in women with preeclampsia, fetal growth restriction and preterm labor [32]. These vessels are of particular importance. In a prospective observational study, placental flow index and vascular flow index were significantly lower in the low gestational age group of women than in normal births. A significantly smaller placental volume was found in the small-for-gestational age group [33].

A growing number of studies suggest that transferring more than one embryo may increase the risk of placental pathology and adverse obstetric outcomes such as preterm birth and low birth weight [34,35]. There is evidence that ICSI is associated with increased anatomical and vascular pathology compared with conventional IVF. Altered expression of imprinted genes has been found in ART placenta [36], and ICSI may result in more abnormalities in histone modifications than conventional IVF [37]. A study by Royster et al. [26] showed that ICSI was associated with unfavorable obstetric outcomes related to placentation, regardless of male infertility status. Sacha et al. [17] have shown that fresh embryo transfer, particularly after ICSI, is associated with increased anatomical and vascular pathology of the placenta, which may contribute to the increased adverse obstetric outcomes in this population of women.

A systematic review and meta-analysis by Chih et al. [38] confirmed that IVF/ICSI pregnancies have a higher risk of hypertensive pregnancy disorders and preeclampsia than pregnancies after spontaneous consumption, regardless of plurality. The risk was particularly high in pregnancies after frozen embryo transfer and egg donation.

The influence of the fertilization method on the subsequent risk of anatomical and vascular pathologies therefore needs further investigation. Some studies even focused on the differences in placental pathology between fresh and frozen ET pregnancies. Mizrachi et al. reported no differences in placental pathology between fresh and frozen ET placentas, while Sacha et al. reported that placental pathology (high risk of marginal cord insertion, accessory lobes, cord abnormalities with associated fetal vascular dysfunction, subchorionic thrombi) and adverse perinatal outcomes such as a large gestational age were more common in pregnancies after frozen ET [39,40].

It would be of great interest to diagnose these placental abnormalities during pregnancy and study their possible effects on the fetus. Unfortunately, studies on ultrasound and magnetic resonance imaging of the placenta during pregnancy have mainly focused on abnormally invasive and low-lying placentas. Few studies have investigated placental morphology, pathology and vascular abnormalities. Identification and description of placental pathology requires a pathologist with expertise in this area. A methodical sonographic assessment of the placenta should include localization of the placenta, visual assessment of the size, thickness, morphology, location and anatomy of the placenta and a search for abnormalities [41]. Increased echogenicity of the placenta may be due to associated placental hemorrhage or hypoxia, and premature calcification may reflect vascular insufficiency and be associated with an unfavorable outcome [42].

The limitations of the studies lie mainly in the different populations. It has also been shown that infertile women are at higher risk of adverse perinatal outcomes regardless of the mode of conception. In addition, all articles should include the American Society of Anesthesiology score (ASA), history of significant illness, body mass index (BMI) and smoking history in the selection criteria. Not all placentas were examined according to the consensus statement of the Amsterdam Placenta Workshop Group, which would allow a more objective assessment and should therefore be used in further studies.

Discovering the reasons for the poorer perinatal outcomes in ART, especially in the fresh ET group, would help identify the high-risk population. By elucidating the nature of placental lesions, we could further explore and define some pathological mechanisms for their management (lower doses in stimulation protocols, use of aspirin, increased pregnancy monitoring, etc.) and reevaluate the criteria for fresh and frozen ET, as freezing practices are currently becoming more common. Further studies will be needed to determine the mechanisms of how ART disrupts placentation and potentially causes adverse outcomes.

Conclusion

Currently, there is evidence that there are more pathological lesions of the placenta in pregnancies after IVF, especially in the ICSI group. There are higher rates of retroplacental and marginal hematoma (p = 0.04), greater thickness (p = 0.02), higher overall rates of vascular and anatomical pathology (p < 0.001) and higher rates of marginal (p = 0.001) and membranous (p = 0.02) umbilical cord insertions than in placentas from pregnancies without ART. Unfortunately, we do not know how anatomical and vascular changes affect the placenta, its function and neonatal outcomes. There is not yet a clear correlation between ultrasound, anatomical and vascular changes in the placenta that correspond to actual findings at postpartum examination. Further prospective studies on different ultrasound features of the placenta compared to anatomical and pathological examination of the placenta after birth are needed.

Authors’ contributions

All authors met the International Committee of Medical Journal Editors (ICMJE) criteria for authorship, contributed to the intellectual content of the study, and approved the final version of the article.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This study was not funded.