A unicornuate uterus is a rare congenital malformation resulting from the abnormal development of the Müllerian ducts. It is classified into two subclasses: a unicornuate uterus with a functional rudimentary horn, either communicating or non-communicating, and a unicornuate uterus without a rudimentary cavity [1]. According to the literature, the prevalence and morbidity rate of unicornuate uterus is 0.3% in the general population [2] and ranges from 5.0–13.0% in all Müllerian anomalies [3, 4]. Typically, a unicornuate uterus tends to be asymptomatic and undetected. While it is well established that pregnancies with unicornuate uterus are associated with various adverse pregnancy outcomes, including infertility, recurrent pregnancy loss, ectopic implantation, preterm delivery (PTD), preterm prelabor rupture of membranes (PPROM), malpresentation, caesarean section (CS), and intrauterine growth restriction (IUGR) [5, 6], the relationship between a unicornuate uterus and obstetric outcomes in the third-trimester has been infrequently studied, and reports of pregnancies reaching term or near term in these cases are rare.

Reduced cavity size and insufficient musculature may lead to uncoordinated uterine contractions, prolonged labor, and an increased rate of CS. Additionally, pregnant women with a unicornuate uterus may be reluctant to accept the risks of uterine rupture, injury from vaginal instrumental delivery, and potential fetal or neonatal death during vaginal delivery. Consequently, obstetricians may be reluctant to engage in cases involving potential medical disputes stemming from adverse obstetric outcomes. Therefore, they may be more inclined to choose CS as the mode of delivery in such situations. Although CS can save maternal and fetal lives, its overuse is associated with increased short- and long-term morbidity and mortality for both mothers and infants [7]. The global CS rate is 21.1% [8]. In China, the CS rate increased from 29% in 2008 to 35% in 2014 [9]. Although the CS rate has decreased with the implementation of the two-child policy, it remains high [10]. Since 2018, the CS rate in urban areas of China has consistently exceeded 40% [11]. With the implementation of the three-child policy in 2021, and in the light of the adverse health effects associated with CS, it is crucial to address the overuse of CS. Obstetricians should perform a risk assessment in pregnant women with a unicornuate uterus prior to delivery. Thus, this study aimed to further evaluate the impact of a unicornuate uterus on maternal and neonatal obstetric outcomes and to compare the results of different mode of delivery, in order to determine the safety and feasibility of vaginal delivery.

Among all Müllerian anomalies, uterine anomalies are most commonly associated with poor obstetric outcomes. Previous studies investigating the impact of a unicornuate uterus on obstetric outcomes have reported higher incidences of PTDs, PPROM, IUGR, malpresentation, and CS [5, 14]. Researchers have proposed various theories on the underlying causes of poor obstetric outcomes. Uterine malformations are linked to a reduced uterine cavity volume, inadequate muscular development, compromised capacity for expansion, irregular myometrial and cervical function, insufficient blood supply, and irregular endometrial growth. These factors may lead to complications in fetal development, pregnancy, and childbirth, increasing the risk of preterm birth and other adverse events for both the pregnant woman and the fetus [15, 16].

Compared to women with a normal uterus, those with a unicornuate uterus had a significantly higher rate of primary infertility in the study. Reduced fertility might be attributed to diminished uterine muscle tissue or a lack of blood vessels, which can lead to inadequate nourishment for the developing fetus [3]. However, the relationship between unicornuate uterus and infertility remains controversial. The causes of infertility may included female factors, male factors, combined factors, and unknown factors [17]. Patients with a unicornuate uterus have a good chance to conceiving through in vitro fertilization and embryo transfer (IVF-ET) or intracytoplasmic sperm injection [18].

The gestational age at delivery in the unicornuate uterus group was lower than that in control group. In individuals with a unicornuate uterus, gestational capacity is compromised due to the presence of only a partial uterine muscle structure [19]. In such congenital conditions, the uterine walls are thinner than those of normal uteri, and myometrial thickness further decreases as the pregnancy progresses, leading to unevenness across the uterine surface [20]. This reduction in muscular support, combined with irregularities in the uterine cavity’s shape, significantly contributes to the risks of both PROM and premature delivery.

Our study found an increased risk of IUGR or SGA in patients with unicornuate uterus, and the birth weight of their babies was significantly lower than that of the control group. IUGR or SGA are similarly attributed to disruptions in uterine blood flow, which may be result from absent or irregular uterine or ovarian arteries, as well as a reduced uterine cavity size [19]. Although there was no significant difference in birth weight between the two groups across different gestational weeks before term birth, following the current American Congress of Obstetrics and Gynecology guidelines for management of IUGR [21], it is advisable to consider serial growth ultrasound examinations to screen for IUGR in pregnancies complicated by maternal Müllerian anomalies.

Higher rates of caesarean deliveries were mainly attributable to nonvertex presentation and excessive concerns among parents. In China, prior to the introduction of the two-child policy, pregnant women with a unicornuate uterus were particularly concerned about the risk of being transferred to an in-labor CS due to the potential threat of uterine rupture. In the study by Fox et al. [14], patients who went into labor and for whom vaginal delivery was attempted showed a statistical trend toward an increased risk of CS, based on the severity of the uterine abnormality. To investigated the relationship between unicornuate uterus and the risk of vaginal delivery, we compare the rates of vaginal births among primiparous women in the study and control groups. However, our study found no association with threatened uterine rupture, preeclampsia, or placental abruption. Importantly, our results showed no significant increase in CS rates among women undergoing a trial of labor. In addition, there was no increased risk of prolonged labor or postpartum bleeding in unicornuate uterus group. NICU admissions did not increase in unicornuate uterus group across different modes of delivery. Erez et al. [22] also addressed the safety of vaginal delivery, noting that they observed no increased incidence of uterine rupture among women with Müllerian duct abnormalities who underwent a trial of labor following a CS. With the implementation of the two-child policy, more pregnant women are choosing vaginal delivery to avoid the adverse consequences associated with CS. This discovery is of significant value, potentially aiding obstetricians in alleviating concerns for women with a unicornuate uterus who are in active labor, by reinforcing their likelihood of achieving a successful vaginal delivery.

A study has reported that a unicornuate uterus is associated with a rudimentary horn in 70–90% of cases [23], which was consistent with our study. The rudimentary uterine horn has historically been recognized as a risk factor for ectopic pregnancies. The rudimentary horn cavity may or may not communicate with the primary uterine cavity. This poses a risk of rupture, as most individuals with a rudimentary horn are asymptomatic and lack a connection to the vagina. For this reason, many obstetricians and gynecologists recommend the preventive removal of the rudimentary horn to mitigate the risk of ectopic pregnancy within the horn [24]. This may also expose women to the risk of uterine rupture during pregnancy. However, Sawada et al. [25] noted that among the 6.6% of patients who underwent rudimentary horn removal, there were no instances of uterine rupture either before or after labor. In a case study detailed by Kanno et al. [26], a woman conceived naturally just four months after laparoscopic removal of the rudimentary horn. The pregnancy proceeded without any complications, culminating in the vaginal delivery of a healthy 3770 g infant at full term, 40 weeks. In our study, two pregnant women who had their residual horn removed due to prior residual horn pregnancies underwent successful vaginal deliveries at 39 and 40 weeks (birth weights 3170 g and 3450 g, respectively). However, the data are very limited, so careful risk assessment is important before attempting vaginal delivery. The mode of delivery should be determined according to the specific circumstances, and fetal heart rate monitoring during vaginal delivery should be strengthened toreduce maternal and neonatal complications.

There are few previous well-controlled studies assessing the risk of uterine rupture before or during labor in pregnant woman with a unicorned uterus. Therefore, the optimal timing and mode of delivery remain uncertain. Vaginal delivery did not increase the risk of prolonging either the first or second stages of labor, nor did it affect the rate of vaginal instrumental deliveries. In the study, the choice of delivery method was determined through consultation between the patient and her doctor. We concluded that patients with a unicornuate uterus did not present an extremely high risk for uterine rupture before labor. However, given the limited number of cases in our study, additional research is necessary to accurately evaluate the risk of uterine rupture in these patients. In our study, out of 94 patients, 21 expressed a desired for vaginal delivery, and 12 of them successfully delivered vaginally without any complications. There was no significant difference in maternal and infant outcomes among the different delivery modes in unicornuate uterus groups. These results indicate that vaginal deliveries are possible, provided that the patients are meticulously assessed for their suitability. Besides, in clinical practice, the possibility of uterine rupture should be considered, with heightened attention to early signs to facilitate the early diagnosis of pre-rupture conditions in the unicornuate uterus or the rudimentary horns.

The strengths of this study include our attempt to explore obstetric outcomes in nulliparous women with unicornuate uterus during the third-trimester. We believe that our research provides significant value to the clinical management of this rare condition. The main limitation of our study is its retrospective design and relatively small sample size; therefore, prospective investigations with larger cohorts are warranted in the future. Another potential weakness is the non-random selection of the control group, which may introduce selection and confounding biases. However, we adjusted for several possible confounders in the multivariate binomial logistic regression analysis model. The conclusions were consistent with the univariate analysis, which guaranteed statistical power.

In summary, a unicornuate uterus is associated with adverse obstetric outcomes for women with singleton pregnancies. These pregnant patients should be informed that they may require proactive and intensive long-term obstetric management. Our study demonstrates that women with a unicornuate uterus can achieve vaginal delivery more successfully following careful evaluation. Unicornuate uterus is not an absolute indication for sections. Physicians should inform patients about the risks associated with labor, establish confidence in vaginal delivery, strengthen labor monitoring, ensure the safety of both mothers and neonates, and improve the success rate of vaginal deliveries.

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LW, YY, JX and XJH designed the research study. LW, YY, YQZ, JX and XJH performed the research and experimental protocols. LW, YY, YQZ and XJH analyzed the data and statistics. LW and XJH wrote the manuscript. LW, JX and XJH made the critical review of the manuscript. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The present study was approved by the Ethics Committee of Women’s Hospital, School of Medicine, Zhejiang University (approval number: IRB-20240274-R), and informed consent was obtained from all the included participants. This study was conducted in accordance with the Helsinki Declaration.

This research received no external funding.

The authors declare no conflict of interest.