Association of Serum Magnesium and Calcium Levels in Patients with and without Diastolic Notch in Uterine Artery Doppler Ultrasonography Imaging in the Second Trimester

Introduction

In normal physiology, uterine circulation varies from high resistance to low resistance during the first half of pregnancy [1]. This change toward low resistance occurs due to trophoblastic cell invasion into the muscular layer of the spiral arteries. This change in the spiral arteries usually begins at week 6 and continues until weeks 22–24 [2]. If there is an increase in resistance in the uterine artery (UA) in these weeks, the trophoblast invasion of the muscular layer of the spiral arteries is insufficient and there is increased resistance in the uteroplacental circulation [3].

The early diastolic notch reflects blood flow amplitude from the uteroplacental circulation with high vascular resistance [1]. These uterine artery notches are known to disappear after 24–26 weeks of gestation in normal pregnancy as the uteroplacental resistance decreases. Therefore, after this period, a persistent diastolic notch, with or without the associated increased S/D ratio, indicates an abnormal uterine artery Doppler finding [1].

UA Doppler USG imaging is a non-invasive method commonly used to assess the blood circulation of the placenta and fetus during pregnancy [4]. The pregnant uterine artery waveform has a specific contour and can be easily differentiated from the external iliac arteries [5]. The results on the clinical value of UA Doppler screening tests vary widely and the correlation of the results varies from 13% to 93%. Diastolic notch refers to an abnormality in blood flow in the UAs and can be observed in the presence of complications such as preeclampsia, intrauterine growth retardation, and pregnancy hypertension (Figures 1, 2). The pulsatility index (PI) is a well-defined marker [6].

Serum Mg and Ca levels play an important role in a healthy pregnancy [7]. About 50% of the serum Ca is in ionized form, 40% is mainly in the albumin-dependent form, and 10% is in the anions [8]. Total serum calcium level is the sum of the total Ca 3 forms, which is least affected by physiological changes in measurement or variations. Therefore, in clinical practice, it is routinely used to represent the state of Ca in the human body [9]. Ca causes vasoconstriction by reducing prostacyclin production and increasing the vasoconstrictor effect of angiotensin II and noradrenaline on the blood vessel wall [10]. It also takes part in vital processes such as regulating muscle functions, nerve conduction, blood clotting, and regulating heart rhythm. Ca exchange takes place between mother and baby during pregnancy; therefore, it is also important for the baby to get enough Ca from the mother [11].

Mg is important for the tone, contractility, and reactivity of blood vessels; therefore, it plays an important role in the physiological regulation of blood pressure. Mg is a cofactor for enzymes and a peripheral vasodilator for smooth muscles [12]. Mg in pregnancy is important in terms of healthy functioning of the nervous system, muscle functions, regulation of blood pressure, and bone development. Mg deficiency can affect the growth and development of the baby, increasing the risk of premature birth [13]. For these reasons, the Ca and Mg levels must be in a healthy balance during pregnancy.

Therefore, this prospective study aimed to evaluate the relationship between serum magnesium and calcium levels and early diastolic notch of the uterine artery by uterine artery Doppler velocimetry in 60 pregnant women in the second trimester of pregnancy.

Material and Methods

ETHICS COMMITTEE APPROVAL:

Ethics approval was obtained from the Istanbul Training and Research Hospital (IEAH) Ethics Committee (decision number dated 22.07.22;227). Informed consent was obtained from patients in writing.

STUDY DESIGN:

This prospective case control study was conducted on a total of 60 patients who applied to Gaziantep Private Bossan Hospital and Istanbul Training and Research Hospital between 31 July 2022 and 28 February 2023. Based on uterine artery Doppler USG, 30 patients were included in the diastolic notch detected group and 30 patients were included in the diastolic notch-free group.

INCLUSION CRITERIA:

We included pregnant women who gave informed written consent, had primigravid and singleton pregnancies between the ages of 19–35 and 18–24 weeks of gestation, and applied to the gynecology clinic.

EXCLUSION CRITERIA:

We excluded women with multiple pregnancies, chronic hypertension, preeclampsia, diabetes, obesity, fetal congenital anomaly, pregnancies with assisted reproductive techniques, pregnant women outside the ages of 19–35, and pregnant women receiving Mg and Ca supplements.

VARIABLES, MEASUREMENTS:

Preliminary data were collected after informed written consent was received from pregnant women at 18–24 gestation weeks, ages 19–35, who agreed to participate in the study, which included detailed history data to evaluate the demographic characteristics of the patients, gestational age, and the condition.

The blood pressure (BP) of the pregnant women that met these conditions and accepted the study was checked after 20 minutes of rest, then bilateral UA Doppler USG was performed using the Voluson™ S8 ultrasound system (Wauwatosa, WI). The procedure is based on obtaining the central sagittal part of the uterus and visualizing the internal cervical os and cervical canal. The converter defines bilateral UAs using color flow mapping. A pulsed wave Doppler with a sampling gate set to 3 mm was used to capture the entire vessel width. The signal was refined until 3 coherent waveforms were obtained. Average values were calculated for PI, resistive index (RI), notch deep index (NDI), and bilateral diastolic notches.

In pregnant women, on the day when UA diastolic notch was detected in Doppler USG, blood samples were taken and other study parameters – serum Ca and Mg values – were analyzed in the laboratory. The laboratory study was conducted at the Istanbul Training and Research Hospital Medical Biochemistry Laboratory and Gaziantep Private Bossan Hospital Medical Biochemistry Laboratory. After 10–12 hours of fasting, samples were collected into yellow tubes without anticoagulants and centrifuged for 10 minutes at 4000 revolutions per minute. Afterward, the serum obtained from centrifugation was studied by photometric method on ROCHE COBAS 501 (California, USA) for serum levels of Ca (photometric pyridoxal phosphate (PLP) method, intra-assay CV: 4.7% inter-assay CV: 8.5% normal range 8.8–10.6 mg/dL) and Mg (photometric pyridoxal phosphate (PLP) method, intra-assay CV: 5.1% inter-assay CV: 7.9% normal range 1.7–2.3 mg/dL). Inappropriate or suspicious results were re-sampled and re-studied.

STATISTICAL METHOD:

Mean, standard deviation, median lowest, highest, frequency, and ratio values were used in the descriptive statistics of the data. Numerical data were assessed for normality before conducting statistical tests. The distribution of the variables was measured by the Kolmogorov-Simirnov test. If the data were normally distributed, an independent t-test was used to compare the characteristics of the 2 study groups. If the data were not normally distributed, the Mann-Whitney U test was performed.

Data analysis was first conducted descriptively for dependent and independent variables. For the analysis of qualitative independent data, the chi-square test was employed, and when chi-square test conditions were not met, Fischer’s exact test was used. The chi-square test was used to analyze qualitative data (in “Education” results, in Table 1). The Fischer precision test is a sub-result of the chi-square test. SPSS 28.0 software was used in the analysis. The P value is considered significant at below 0.05. The results and the tests are displayed in Tables 1 and 2, and the following discussion.

Results

CHARACTERISTICS OF ALL PATIENTS:

In this study, the average age of pregnant women was 25.7±3.67 and the mean body mass index (BMI) was 25±4.02. The pregnant women participating in the study did not have any history of disease or drug use. Of the 60 pregnant women included in the study, 30 had a diastolic notch (+), and 30 did not (−) in UA Doppler USG. These pregnant women were considered as 2 different groups. The average serum Mg value was 1.77 (±0.26) mg/dL, and the average serum Ca value was 9.1(±0.63) mg/dL in both groups (Table 2). Table 2 shows the demographic characteristics of the patients participating in the study, so no test has been applied to this table.

COMPARISON BETWEEN PREGNANT WOMEN WITH AND WITHOUT DIASTOLIC NOTCH DETECTED IN THE UTERINE ARTERY IN DOPPLER VELOCIMETRY IN THE SECOND TRIMESTER OF PREGNANCY:

The BMI values of the patients did not differ significantly (P=0.206) between the groups with and without diastolic notch. The educational status in the diastolic notch and non-diastolic groups did not differ significantly (P=0.739) (Table 1). The gestational week values did not differ significantly (P=0.211) between the diastolic notch and non-diastolic groups (Table 1). In the diastolic notch group, the Mg value was significantly lower (P=0.048) than in the non-diastolic notch group. In the diastolic notch group, the Ca value was significantly higher than in the non-diastolic notch group (P=0.002) (Table 1). In the diastolic notch group, the Mg level was 1.72±0.16 mg/dL and the Ca level was 9.3±0.69 mg/dL, while in the non-diastolic notch group, the Mg level was 1.82±0.32 mg/dL and the Ca level was 8.8±0.41 mg/dL (Figure 3A, 3B). Preeclampsia developed in the later stages of pregnancy in 6 (20%) patients with diastolic notch.

Discussion

THE LIMITATIONS OF THE STUDY:

The most important limitation of the study is the small sample size. Additionally, albumin level was not recorded. This situation may affect the serum calcium level. However, we enrolled healthy pregnant women without any comorbidities. This may suggest that the serum albumin level was not significantly different. In addition, other mineral and vitamin levels were not studied except calcium and magnesium. The parameters not studied may also potentially affect the formation of diastolic notch in the uterine artery.

Conclusions

In this study, the relationship between the presence of diastolic notch with serum Mg and Ca values was investigated. Our findings show that the serum Mg value was significantly lower and the Ca value was significantly higher in the diastolic notch group. The significantly lower serum Mg and higher Ca value seen in the diastolic notch group are potentially related to the presence of the diastolic notch. However, studies with larger samples are needed.