Impact of Blood Urea Nitrogen and Creatinine Levels on Maternal and Fetal Outcomes of Pregnancy: a Retrospective Cohort Study

Guifang Deng (  misy y@163.com ) Union Shenzhen Hosipital of Huazhong University of Science and Technology https://orcid.org/00000002-9520-2793 Lanlan Wu Union Shenzhen Hospital of Huazhong University of Science and Technology Yao Liu Union ShenzhenHospital of Huazhong University of Science and Technology Zengyou Liu Union Shenzhen Hospital of Huazhong University of Science Technology Hengying Chen Shantou University Medical College Siwen Shen Union Shenzhen Hospital of Huazhong University of Science and Technology Yuanhuan Wei Union Shenzhen Hospital of Huazhong University of Science and Technology Ruifang Sun Union Shenzhen Hospital of Huazhong University of Science and Technology


Background
In the process of pregnancy, the renal workload increases due to rise in maternal and fetus metabolites [1]. These physiological changes may lead to renal pathological damage and renal insu ciency, and the symptoms are relatively insidious [2][3][4]. A series of studies suggested that signi cant impairment of kidney function was independently associated with poor obstetric outcomes particularly preterm birth and growth restriction [3,[5][6][7][8][9][10]. It has been recently reported that pregnancy women in the early stage of chronic kidney disease (CKD) orwith a mild decrease in glomerular ltration rate (GFR) were at an increased risk of adverse maternal and fetal events [5]. However, Kidney disease in pregnancy is often under-recognized and under-appreciated owing to the lack of symptoms and routine kidney function testing in pregnancy. Therefore, close monitoring of renal function during pregnancy plays an important role in reducing the risk of adverse maternal and infant events related to renal function.
BUN is the main end product of protein metabolism. The deamination of amino acids produces NH 3 and CO 2 , which are synthesized into urea in the liver and then lter out of the glomeruli and excrete in the urine eventually [6]. BUN is used as a parameter to evaluate renal function since it will increase when renal e ciency is decompensated. A series of animal and epidemiological researches indicated that elevated BUN levels was a risk factor for insulin sensitivity reduction and increased the risk of diabetes and GDM [7][8][9]. However, the impact of BUN on fetal and maternal outcomes has not been established.
SCr, another indicator to assess renal function, is the product of muscle metabolism. In Muscle cells, creatine slowly forms SCr through irreversible non-enzymatic dehydration reaction, which is released into the blood and excreted by the kidney [6]. A recent large population-based cohort study in non-GDM pregnant women aged 18-45 years showed that higher SCr levels were related to decreased risk of macrosomia [10]. All told, there were limited data in the literature on the association between adverse pregnancy outcomes and maternal BUN and SCr levels. No research has paid attention to the in uence of the change values during pregnancy and the combination of the two indicators on the pregnancy outcome as well.
In the present study, we aimed to examine the association of BUN and SCr in the second and third trimesters of pregnancy with adverse pregnancy outcomes including premature rupture of membranes (PROM), macrosomia, small for gestational age infants (SGA), large for gestational age infants (LGA), and to evaluate the in uence of the combination of the two indicators and the changes between those two indicators in the second and third trimester of pregnancy on maternal and infant complications in a retrospective cohort study.

Study design and participants
A total of 1,701 pregnant women who registered and attended for their routine rst hospital visit in pregnancy at Antenatal Department of Union Shenzhen Hospital of Huazhong University of Science and Technology (Shenzhen, Guangdong) and planned to give birth at this hospital were recruited from January 2015 to December 2018. The following exclusion criteria were included: history of liver disease (n = 45), diabetes or hypertension (n = 13), kidney disease (n = 5), heart disease (n = 5), twin or multiple pregnancy (n = 27). At last, a total of 1,606 gravidas with singleton pregnancies were included in the present study. The basic information of the participants was collected at the beginning, and serum BUN and SCr were measured in the second (16-18 th ) and third (28-30 th ) trimester of gestation respectively. And follow them up until delivery. Participants signed the informed consent at the beginning of the study with all procedures have been approved by the Ethics Committee of the Union Shenzhen Hospital of Huazhong University of Science and Technology.

Collection of baseline information
Age (years), education (primary, secondary, college or above), smoking status (yes or no), alcohol status (yes or no), conception method (natural or arti cial), parity (primiparity or multiparity), history of miscarriage (yes or no), embryo number and history of disease (e.g., liver disease, diabetes or hypertension, kidney disease, heart disease) were obtained through face-to-face interviews by a welltraining investigator and questionnaires were completed simultaneously. Height and weight were measured using a electronic scale with detailed instructions to follow: stands up straight after taking off shoes with shoulder when she taking off shoes, stands up straight, shoulders parallel and the body is naturally relaxed and accurate to 0.1cm and 0.1kg respectively. Pre-pregnancy body mass index, BMI (kg/m 2 ) was calculated as weight (kg) divided by the square of the height (m 2 ).

Laboratory assays
At 16-18 th weeks and 28-30 th weeks of pregnancy, fasting venous blood were collected by professionally trained investigator. The samples were centrifuged at 3,500 rpm for 5 minutes at 4°C within 2 hours of collection. The BUN and SCr level were measured by enzymatic assay. All laboratory measurements were performed using accelerator a3600 automatic analyzer (Abbott, Chicago, USA).

Assessment of pregnancy outcomes
The adverse pregnancy outcomes of this study include: PROM, macrosomia, SGA and LGA. We followed the de nition of the International Classi cation of Diseases, 10th Revision (ICD-10). The diagnostic criteria for PROM was that maternal self-report of vaginal uid ow, vaginal PH > 6.5 on litmus test paper, and amniotic uid components on vaginal smear [11][12][13]. Macrosomia was diagnosed as a newborn with a birthweight greater than 4000g [14]. SGA and LGA were generally de ned as below the 10 th or above the 90th percentile on the growth chart, respectively [15].

Statistical analyses
Baseline information was presented as means (SD) for continuous variables and proportion (%) for categorical. Statistical difference between second trimester and third trimester were tested using Student's t-test. BUN and SCr were categorized by quartile distribution with the rst quartile serving as the reference. Odds ratios (ORs) and 95% con dence intervals (95% CIs) were calculated by using logistic regression models to examine the association of BUN and SCr of gestation with the risk of adverse pregnancy outcomes across each of the quartiles. Then, the differences (d-value) of the levels between BUN and SCr in the third and second trimester was calculated and analyzed in order to illustrate whether the risk of adverse pregnancy outcomes was caused by changing in concentration during pregnancy. Two models were included in the present study: Model 1 was unadjusted; Model 2 was adjusted for age, education, smoking status, alcohol status, conception method, parity and history of miscarriage. Restricted cubic spline(RCS) was used to re ect the nonlinear associations of BUN and SCr and d-value with adverse pregnancy outcomes. The data were regrouped based on the "cut-off value " of BUN and SCr in the RCS, Group1: no indicator exceeded the "cut-off value"; Group2: one of the indicators exceeded the "cut-off value"; Group3: indicators exceeded the "cut-off value", and the above two models were also constructed. Finally, a age-subgroup (< 35 and ≥ 35 years) was performed because advanced maternal age is a known risk factor for adverse pregnancy outcomes [16]. All analyses were carried out by using SPSS 24.0 (SPSS Inc., Chicago, IL, USA) and a two-sided P-value < 0.05 was considered statistically signi cant. Graphic production was completed by using R version 3.0.3 (The R Foundation for Statistical Computing, Vienna, Austria).

Baseline characteristics
A total of 1606 singleton pregnant women aged 31.59 (3.83) years were included in the study with 308 cases of PROM, 95 cases of macrosomia, 65 cases of SGA, and 237 cases of LGA. As presented in Table 1, 383 cases were elderly pregnant women. The differences of BUN and SCr between the second trimester and third trimester were statistically signi cant (P < 0.05) and the levels of BUN and SCr in the third trimester were higher than those in the second trimester. LGA 237 (14.8) Data was presented as mean(SD) for continuous variables and n(%) for categorical. Statistical difference between second trimester and third trimester were tested using Student's t-test. Abbreviations: BMI: body mass index; BUN: serum urea nitrogen; SCr: serum creatinine; PROM: premature rupture of membranes; SGA: small for gestational age; LGA: large for gestational age. a p < 0.05.

Association of BUN and SCr with adverse pregnancy outcomes
The associations of BUN and SCr in the second trimester with adverse pregnancy outcomes were shown in Table 2 and Additional le Fig. 1. The second trimester BUN and SCr levels were not signi cant associated with PROM, macrosomia, SGA, or LGA. However, maternal with SCr levels in the fourth quartile had a 45% (95% CI, 1.01-2.09) higher risk of PROM than those in the rst quartile. And each standard deviation (SD) of SCr levels increased the risk of PROM by 16% (95% CI, 1.02-1.32). Moreover, Additional le Fig. 1 indicated that the BUN level between 26.80 to 43.04µmol/L was a protective factor to PROM. The associations of BUN and SCr in the third trimester with adverse pregnancy outcomes were shown in Table 3 and Additional le Fig. 2 Fig. 2 showed that when SCr levels between 28.30 to 46.80µmol/L, it was a protective factor to SGA, but inversely related with LGA risk in a non-linear manner. Third trimester SCr levels was not statistically signi cant associated with risk for PROM. The associations of changes for d-value of BUN and SCr with adverse pregnancy outcomes were shown in Table 4 and Additional le Fig. 3. D-value of BUN levels were associated with the risk of SGA and LGA.  D-value of SCr levels were associated with the risk of macrosomia and LGA. Compared with those in the rst quartile of d-value for SCr levels, OR 95% CI of macrosomia in the third quartile was 0.43 (0.22-0.82), OR 95% CI of LGA in the third quartile was 0.49 (0.32-0.74). Additional le Fig. 3 showed that the d-value of SCr level from − 13.40 to 3.65mmol/L was inversely associated with the risk of both macrosomia and LGA in a non-linear manner. The associations of d-value of SCr in the third trimester with PROM and SGA were not statistically signi cant.
Association of combined classi cation of BUN and SCr with adverse pregnancy outcomes The associations of combined classi cation of BUN and SCr with adverse pregnancy outcomes were shown in Table 5.

Discussion
In this retrospective cohort study, we observed an increased risk of PROM in pregnant women with high or even those towards the upper limit of the normal range of SCr levels during 16-18th weeks of gestation.
During 28-30th weeks of gestation, pregnant women with higher or even those towards the upper limit of the normal range of BUN and SCr exhibited an increased risk of SGA. The elevated changes of BUN during pregnancy in uenced the birth weight of newborn. In addition, the risk of SGA increased substantially if BUN and SCr levels high or even those towards the upper limit of the normal range simultaneously.
Mild renal function damage would lead to improper regulation of gestational adaptation to volume and vascular pressure change, resulting in obstetric complications [17]. And as kidney function declines, the chance of a women having adverse maternal outcomes increased [18]. give an early warning of occurrence for SGA. Renal disease is often clinically silent in the early and middle stage of renal injury. Bun and SCr may change only slightly until GFR drops by more than 50%.
However, these pathological changes affect the regulation of blood pressure and blood volume, which can easily lead to insu cient uterine placental perfusion and affect fetal development. The exact mechanism associated with this issue was not well understood and requires further studies.
The increase of an index may be greatly in uenced by external factors, thus we combine the BUN and SCr measurements and regroup them for analysis. The risk of SGA increased by 249% in pregnant women whose two indicators exceeded the cut-off value, compared with those whose two indicators did not exceed the cut-off value. We also analyzed the relative BUN and SCr change from the second trimester to the third trimester of gestation age. According to the Table 5 and Supplementary Fig. 3, the risk of SGA was higher with a elevated meteral BUN level of > 0.64mmol/L. Therefore, not only the real-time level of renal function indicators must be paid attention to, but also the dynamic monitoring of renal function. A interaction and subgroup analysis by age (< 35 and ≥ 35 years) was also performed to verify whether age would affect the risk of adverse pregnancy outcomes in association with maternal BUN and SCr levels.
We found a signi cant correlation between BUN and SCr levels and adverse pregnancy outcomes in women aged < 35 years, but not in women aged ≥ 35 years. There was no signi cant interaction between maternal age and the level of BUN and SCr (all P-interaction > 0.05). Further studies with larger sample sizes are required to clarify the risk of adverse pregnancy outcomes in relation to BUN and Scr levels and age dependent effects.
Although our study comprehensively explored the association between maternal renal function using two parameters and the risk of adverse pregnancy outcomes in a relatively large sample size, some limitations in this study remain. Firstly, the analytic cohort were from China, which may limit the generalizability of the study results. Secondly, although we accounted for known confounders, some unmeasured or unknown residual confounders remained (either unmeasured or unknown). Finally, the concentrations of BUN and SCr were greatly in uenced by other factors, and did not change signi cantly when GFR was slightly decreased, so they were not sensitive indicators of renal damage. However,the chosen biochemical parameters of BUN and SCr to assess maternal renal function are simple, inexpensive and readily available tests thus should be additionally evaluated [23].

Conclusions
Higher BUN and SCr levels during the 28-30th week of gestation even those towards the upper limit of the normal range can act as a warning sign of the impending SGA. Elevated changes of BUN and SCr during pregnancy also associated with the lower birth weight. Combined analysis of the two indicators is more conducive to evaluate renal function and prevent the occurrence of SGA. More attention should be paid to the dynamic monitoring of renal function during pregnancy.

Consent for publication
Not applicable Availability of data and materials The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.