In this study, 1200 pregnant women were randomly selected. The maternal age increased significantly with the increase of parity. The proportion of women whose age ≥ 35 years (elderly parturient women) in parity-3 was up to 41.3%, which was nearly 6 times larger than that in parity-1. Advanced age is a key factor that results in adverse pregnancy outcomes and affects the way of delivery [1-3]. Thus, conceiving at an appropriate age is a beneficial way to reduce the burden of puerpera, fetus, and society. With increasing the times of birth, the maternal BMI increases, as well as the proportion of obesity. This phenomenon is similar to that reported by Paulino et al. [4-6]. However, our study showed that there is no obvious relationship between gestational weight gain and parity, indicating that the difference in BMI between the groups is not caused within a few months of pregnancy, which may be caused by the long-term metabolic changes in the body after each delivery. Our results showed that the rate of scar uterus increased significantly with the increase of the number of deliveries, implying that people's health awareness is increasing. More women receive uterine myomectomy in an early stage of the disease, which leads to scar uterus. However, the main reason is still the widespread development of cesarean section. In 2010, the World Health Organization (WHO) reported that the cesarean section rate in China was as high as 46.2% [7]. The rate of cesarean section still increased after the opening of the two-child policy in China, and some hospitals even reached 70-80%. In this study, the cesarean section rates were 22.3%, 41.8%, and 65.8% in parity-1, parity-2, and parity-3, respectively, which far exceeded the warning line of 15% set by the WHO. Many doctors and patients still stay in the concept of “once section, always section” for fear of uterine rupture, which is a major reason for cesarean section rate increasing with the increase of parity. Fortunately, it has been 40 years since the American Association of Obstetricians and Gynecologists (ACOG) published the first edition of TOLAC guidelines in 1982. The global research on the safety and influencing factors of VBAC has never stopped. A lot of studies [8, 9] showed that the maternal and infant outcomes of scar uterine patients within the trans-vaginal delivery after excluding contraindications are far superior to those of multiple elective cesarean sections. Even for uterine rupture, the incidence is negligible compared with the adverse outcomes caused by cesarean section, as well as the rising rate of successful trial delivery, resulting in a gradual downward trend of the cesarean section rate. In this study, 2, 9, and 14 women in parity-1, parity-2, and parity-3 underwent vaginal delivery after a previous uterine myomectomy, a previous cesarean section, and a scar uterus, respectively. We should note that 2 of whom in group parity-3 underwent vaginal delivery after a second cesarean section. Fortunately, none of these 25 women had uterine rupture. Millier et al. [10] performed trial labor in 12,707 pregnant women with a history of cesarean section, and he found that the uterine rupture rates of 1 and 2 times was 0.6% and 1.8%, respectively. The probability of uterine rupture is much lower than that of adverse maternal and neonatal outcomes caused by cesarean section. Therefore, the development of scar uterus trans-vaginal trial birth should be encouraged in hospitals with multidisciplinary cooperation and emergency surgery conditions. Once uterine rupture occurs, cesarean section should be performed immediately to reduce maternal and neonatal mortality.
There have been several studies on the association between parity and diabetes, epilepsy, dementia, bone mineral density, cancer, and metabolic diseases [11-18]. However, the effects of parity on maternal and infant pregnancy outcomes are poorly reported until now. In this study, the results showed that parity was a risk factor for preterm birth, perinatal anemia, and uterine rupture after controlling for other confounding variables. The risk of preterm birth was 1.602 times higher than the previous birth. A possible reason for this is that the uterine cavity was dilated with the increase of parity, resulting in preterm birth with fetal membrane integrity. Moreover, with the increase in the number of deliveries, due to maternal advanced age, scar uterus and other high-risk factors that prevent the continuation of the pregnancy, the termination of pregnancy by induction of labor or cesarean section at less than 37 weeks leads to therapeutic preterm birth. The risk of perinatal anemia increased 1.468 times with each additional birth. Shah et al. [19] concluded that the risk of anemia during pregnancy was increased by high parity, and the incidence of anemia increased with increasing the number of pregnancies. Previous reports [20, 21] suggested that the incidence of anemia varies with country and the developing countries have a higher ratio than the developed countries. The probability of developing perinatal anemia varied significantly among the three maternal groups in this study, while it was significantly less than that (56%) reported by Saghafi et al. [22] ten years ago. The living conditions are improving and the prenatal examination is also popularized in backward areas. The harm of anemia to mothers should not be underestimated, low haemoglobin concentrations during pregnancy can be associated with an increased risk of maternal and perinatal mortality and low size or weight at birth [23]. Thus, all the women whom planning to get pregnant should carry out corresponding examinations before pregnancy and in early, middle and late pregnancy, and the iron and vitamins should be added when its necessary. It is known that the uterine rupture have been associated with cesarean section, while the relationship between parity and uterine rupture is rarely reported. Our results indicated that the risk of uterine rupture was 2.752 times higher than the previous birth. The multiple pregnancies will lead to uterine expansion, a thin muscle layer, and a decrease in elasticity of the uterine wall. As a result, the uterine will be ruptured in the middle of pregnancy, late pregnancy, and childbirth by the increased intrauterine pressure. Meanwhile, the strength of uterine contractions will increase with the parity, and thus the uterine rupture may result from the strong uterine contraction.
On the other hand, our results revealed an unexpected phenomenon that parity is a protective factor. The risk of low birth weight infants, amniotic fluid turbidity, and fetal distress decreased with increasing of parity. A study of 29 countries in Asia, Africa, and Latin America showed that pregnant women without a reproductive history had a greater risk to have a small for gestational age (SGA) infant [24]. In our study, the ratios of low birth weight infants in parity-1, parity-2, and parity-3 were 6%, 1.3%, and 2%, respectively. These results are consistent with that reported by Chen et al. [25], which showed that the ratio of low birth weight infants in primipara was 6.1% in the Chinese mainland area. With increasing the number of parity, the uterus expands and the blood flow of the uterus and placenta become greater. As a result, the fetus can get abundant oxygen and nutrition and the risks of low birth and hypoxia reduce significantly. It is well known that amniotic fluid turbidity and intrauterine distress interact with each other, while some Chinese scholars have pointed out that the most important factor affecting meconium excretion is gestational age, the greater the higher the probability of amniotic fluid meconium contamination [26].
Our results show that there was no significant difference in the ratios of placenta previa among the three groups. However, it is worth noting that the occurrence of placenta accreta in parity-3 was generally accompanied by the placenta previa, which increases the difficulty of surgery dramatically. In this case, clinicians should focus on it. Surprisingly, the incidence of hypertensive disorders in pregnancy and postpartum hemorrhage did not increase with the increase of parity. With more standardized prenatal examinations, diseases during pregnancy can be treated early. Supplementing the vitamin and calcium timely can play an important role in preventing hypertension disorders for pregnant women. At the same time, the test of hemodynamics can screen out the high-risk groups, and then some special interventions, such as weight management and aspirin, can be used to reduce the incidence of hypertensive diseases during pregnancy. Although the difficulty of surgery has increased with the number of cesarean sections, the incidence of postpartum hemorrhage has been significantly controlled with the continuous improvement of medical care in China. Drug hemostasis, uterine cavity tamponade, uterine artery ligation, uterine compression sutures, and other methods have been used to prevent and /or control postpartum hemorrhage effectively. In addition, there were significant differences in the occurrence of PROM, placenta implantation, gestational diabetes mellitus, uterine inertia, ICP, and abdominal adhesion among the three groups when compared by parity. However, the parity was not an independent factor and its effects were related to age, BMI, conception mode, and scar uterus. Egeland et al. [27] reported that the risk of gestational diabetes mellitus of the primipara was significantly higher than that of mutipara. While, our results showed that the parity was not a factor affecting gestational diabetes after adjusting for other confounding influencing factors, such as age and BMI. The maximum Youden index assessing hazard bounds was calculated according to the ROC curve, as shown in Fig. 3. The results showed that women with age ≥ 33.5 years and/or BMI ≥ 27.00 were the main population of gestational diabetes mellitus.