Comparative sonographic evaluation of placental thickness in pregnancy-induced hypertension and normotensive pregnant women in the University of Calabar Teaching Hospital, Calabar, Nigeria

INTRODUCTION

Pregnancy-induced hypertension (PIH) occurs as a direct result of a gravid state. It consists of gestational hypertension, which is defined as a de novo increase of the systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg in a pregnant woman of gestational age ≥20 weeks and pre- eclampsia, which is the de novo occurrence of proteinuria ≥300 mg/24 h or ≥30 mg/mmol or ≥2+ on the test strip, in a single urine specimen, in an individual with gestational hypertension.^[1-4]

The placenta is a highly vascular fetomaternal organ which maintains the maternal-fetal circulation and fetal nutrition through its connection with the umbilical cord. The elimination of fetal waste products to prevent the accumulation of toxins is also achieved through this connection. Importantly, the placenta produces a growth- stimulating hormone that promotes fetal growth.^[5,6]

Placental thickness (PT) increases with fetal age and a normal growing placenta in terms of structure and function produces normal growth and development of the fetus. However, in PIH there is impairment in the development of the placenta, which consequently has a profound negative impact on the PT and, by extension, the fetal development, growth and pregnancy outcomes. The outcome of PIH is initially, the reduction of the placental size and thickness below the normal expected value and, eventually, a reduction in the weight of the fetuses of affected women.^[7,8]

PIH with other hypertensive disorders in pregnancy contributes up to 9.1% of the maternal deaths in Africa, 16.1% in developing countries as a whole, 9.1% in Asia, and 25.7% in Latin America and the Caribbean.^[9] Between 1991 and 1999, pregnancy-related hypertension caused 15.7% of maternal deaths in the United States.^[4,9]

Ultrasonography is a rapid and apparently safe method of evaluating the thickness of the placenta and the fetus.^[8] This imaging modality is safe because it utilizes non-ionizing radiation, which is devoid of teratogenic potential in the fetus and maternal cancer induction. Using this modality, the placenta can be seen in its entirety, the point of cord insertion noted, and the thickness (placental) measured.

The aim of this study is to sonographically evaluate the effect of PIH on PT and, subsequently, the fetal weight. We had to measure the body mass index (BMI) to see if it will influence the PT or EFW.

MATERIAL AND METHODS

The study was a 12-month prospective cross-sectional case– control study conducted in the Department of Radiology, University of Calabar Teaching Hospital (UCTH), Nigeria, from November 2015 to October 2016. Ethical approval for the research was obtained from the health research ethics committee of the UCTH, and informed consents were given by the pregnant women through an endorsed document (consent form). Consenting women were administered questionnaires.

The sample size was obtained using the statistical formula of Fischer;^[10]

Equation 1:

Where

n is sample size

z is standard normal deviate set at 1.96 corresponding to 95% confidence level

p is the prevalence of the hypertension in a previous study which is 6.96%^[11]

q = 1-p

d is the degree of accuracy desired which is set at 0.05

Therefore, n = {(1.96)² × 0.0696 × 0.9304}/{0.05}²

n = 99.51.

Hence, the sample size was approximated to 100 women. Employing a 1:1 ratio, the cases were 100 (hypertensive pregnant women), and the control was 100 (normotensive pregnant women). The two groups were gestational age- matched.

Pregnant women below 40 years of age^[12] with singleton gestation between 20 and 40 weeks were referred for this study from the antenatal clinic (ANC) of the Department of Obstetrics and Gynecology, UCTH. A curvilinear transducer operating between 3.5 and 5 MHz connected to a 2 Dimensional Aloka prosound SSD-3500SX fitted with Doppler facility (manufactured in 2008 by Aloka Company limited located in Meerbusch, Germany) was used for the sonographic evaluation of the women.

RESULTS

The maternal age for all the women in both groups ranged from 15 to 39 years. Most of the participants (70%) were between 25 and 34 years, while the 15–19 year group was the least in frequency, as shown in Figure 2.

Significant statistical differences were observed in the PT, maternal blood pressure, and BMI between the hypertensive and control groups. These were not demonstrated for the EFW and maternal age between the groups. Details are presented in Table 1.

There was a linear increase in the PT with gestational age from the 20^th to the 40^th in the control group but there were downward trends in the mean value in 7 out of the 20 weeks that were assessed in the hypertensives. These observations are outlined in Table 2.

Close

Figure 1:

Color flow Doppler image showing the placental thickness measurement plane. The placental thickness is the distance from A to B, where A = The point of insertion of the umbilical cord at the chorionic surface of the placenta and B = The basal surface of the placenta.

Export to PPT

Close

Figure 2:

A bar chart showing the frequency of the different age groups in the hypertensives and the control.

Export to PPT

Figure 3 shows the number of PIH cases who did or did not develop proteinuria while having gestational hypertension and to what degree. No proteinuria was observed among the normotensives and 71 of the PIH. Most of the proteinuric had either + or +++ in about the same frequency (n = 10).

There was a significant correlation between PT and the EGA and EFW in both groups (P = 0.000). Furthermore, PT showed significant correlation with proteinuria (P = 0.011) among the hypertensives and with BMI (P = 0.007) in the control [Tables 3 and 4].

Figure 4 demonstrates the changes that occur in the PT as the degree of proteinuria rises.

DISCUSSION

The results obtained in this study demonstrated that the PT of the PIH subjects was significantly reduced when compared to the normotensive pregnant women, but there was no correlation with the maternal systolic or diastolic blood pressure in both groups. It was also observed that the PT in the PIH group had a significant negative correlation with the degree of proteinuria (P = 0.011). The implication of this finding is that elevated blood pressure, irrespective of its severity, was not the direct cause of the PT reduction in the PIH but the appearance of proteinuria. As the degree of proteinuria increases, the PT reduces. Baghel et al.^[5] in India had similar findings to this study as they noted that pre- eclampsia resulted in a reduction in PT. However, Afrakhteh et al.^[6] in Iran and Miwa et al.^[13] in Japan had contrary observations of increased PT in pre-eclampsia and PIH patients, respectively. Their explanation for this was that PIH produced inflammation of the placenta and thus, increase in its thickness.

Close

Figure 3:

Frequency distribution of the degree of proteinuria.

Export to PPT

Close

Figure 4:

A bar chart demonstrating the relationship between placental thickness and urinalysis (± proteinuria).

Export to PPT

This index study reveals that the degree of proteinuria had a significant negative correlation with EFW (P = 0.047), maternal age (P = 0.027), and parity (P = 0.047) but a very strong positive correlation with the systolic (P = 0.004) and diastolic (P = 0.001) blood pressure. Thus, it can be assumed that when younger women with a lower parity, especially the teen primigravida/nulliparous develop elevated blood pressure in pregnancy, it rapidly leads to proteinuria (pre-eclampsia). Moreover, the increase in the degree of proteinuria has the propensity to reduce the fetal weight in the course of pregnancy and inevitably a low birth weight at delivery.

Peter et al.^[4] corroborated the above findings when they observed a significant positive correlation between proteinuria and blood pressure in their study and that pre- eclampsia was more common in first pregnancies. Singh et al.^[14] in Sokoto also reported that nulliparous women were at increased risk of having PIH and pre-eclampsia and that women with pre-eclampsia delivered low birth weight babies while normotensive pregnant women and PIH (devoid of proteinuria) delivered normal-weight babies. Yang and Wang^[15] had findings that supported this study in their research. Pegu et al.^[16] also found that pre-eclampsia was high in primigravid women below 24 years of age among the 200 pre-eclampsia studied.

A significant positive correlation between the PT and EFW was observed in this study in both groups (P = 0.000). Studies carried out by Ohagwu et al.^[7] and Karthikeyan et al.^[17] in India and Adeyekun and Ikubor^[18] also in Nigeria all had similar findings. This clearly demonstrated that any factor which affects the PT will equally have a corresponding effect on the fetal weight.

Although the PT was significantly lower in the PIH cases due to the superimposed proteinuria, the mean fetal weight in the PIH cases was paradoxically higher than the mean fetal weight in the normotensive pregnant women. This could be due to the higher mean value of the BMI in the PIH in relation to its value in the control group. Erika et al.^[19] in agreement with this view stated in their Vietnam based study that a low BMI and suboptimal weight gain during pregnancy is recognized as a major factor responsible for the delivery of infants that are small for gestational age. However, Vietnamese women tend to gain less weight when pregnant, so weight gain could actually be race-related.^[19] This study reflected the importance of maternal BMI on the ultimate fetal weight on delivery even in the face of the EFW, reducing potentials of pre-eclampsia. In other words, pre- eclampsia in the background of a low BMI (underweight) will greatly reduce the fetal weight more than it would in a pregnant woman that has a normal or high BMI (overweight or obese).^[19]

A significant positive correlation existed between BMI, PT, and systolic blood pressure in the control group, but no such relationship was present in the PIH group. Probably in PIH with subsequent development of proteinuria (pre-eclampsia), the relationship between BMI and PT and the systolic blood pressure become altered. Peter et al.,^[4] however, demonstrated a significant positive correlation between BMI and hypertension and further observed that a BMI >30 was associated with hypertension in pregnancy. Middendorp et al.^[1] corroborated this finding and were of the opinion that an elevated BMI will lead to a rise in the level of serum cholesterol and consequently cause elevation of the blood pressure in affected pregnant women.

The limitations identified in this research included the fact that the gestational age at the onset of hypertension in all the pregnant women could not be ascertained since a reasonable number did not commence ANC before 20 weeks of gestation. Moreover, few women ignore the necessity of routine blood pressure assessment to detect hypertension before conception.

Another limitation is that the research was only carried out in one health facility in Calabar, which will suggest that its findings are only a mirror of what was obtainable where the study was conducted. Furthermore, since it was a cross- sectional study, the evaluation was done once, whereas a longitudinal study could have been a platform to monitor the changes, through serial measurements, in the PT and the fetal weight as the blood pressure and proteinuria varied.

CONCLUSION

Based on the observations in this study, it may be concluded that the PT is lower in pregnancy-induced hypertensives, and this occurs when there is secondary proteinuria superimposed on gestational hypertensives. It is recommended that a study be done exclusively on pregnant women with isolated proteinuria in the absence of PIH, chronic hypertension, UTI, gestational diabetes, and other systemic diseases that can trigger proteinuria. This will afford the opportunity to objectively assess the solitary effect of proteinuria on PT.

A high maternal BMI appears to have a buffering effect on the fetal weight reduction tendency of proteinuria in pregnancy. It is recommended that a study be done involving normotensive pregnant women and proteinuric PIH women who have been BMI matched and gestational age matched, like in the index study, to eliminate the interference of BMI in the analysis.