Role of Measurement of Placental Thickness and Diameter at the Third Trimester Using Two Dimensional Ultrasound in Determination of Low Birth Weight

INTRODUCTION

Estimation of fetal weight is essential in our daily obstetric practice, especially in the third trimester. It guides obstetricians to make up their decisions as regard time and mode of delivery to guard against complications of low birth weight and macrosomic babies during labor and puerperium. ^{1, 2}

The definition of fetal growth restriction (FGR) is not simple and is usually based on birth weight 3

SGA definition is based on the cross-sectional evaluation (either prenatal or postnatal), and this term has been used for those neonates whose birth

weight is less than the 10^th percentile for that particular gestational age or tow standard deviations below the population norms on the growth chart, and the definition considers only the birth weight without any consideration of the in-utero growth and physical characteristic at birth. ⁴

IUGR is a clinical definition and applies to neonates born with clinical features of malnutrition and in-utero growth retardation irrespective of their birth weight percentile. Henceforth, appropriate for gestational age (AGA) infants can be labeled as IUGR if they have features of in-utero growth retardation and malnutrition at the time of birth. Therefore, it is important to keep in mind that neonates with a birth weight less than the 10^th percentile will be SGA, but not an IUGR if there are no features of malnutrition, and a neonate with a birth weight greater than the 10^th percentile will be an IUGR despite being an AGA if the infants have features of malnutrition at birth. ⁴

Risk factors for fetal growth restriction include constitutionally small mothers, poor maternal nutrition, social deprivation, maternal and fetal infections, congenital malformations, chromosomal aneuploidies, disorders of cartilage and bone, drugs with teratogenic and fetal effects, vascular disease, renal disease, pregestational diabetes, chronic hypoxia, anemia, placental and cord abnormalities, infertility, extrauterine pregnancy, anti-phospholipid antibody syndrome, genetics and multiple fetuses. ⁵

The placenta links mother and fetus by indirect interaction with maternal blood to allow the exchange of gases and nutrients with fetal capillary blood within connective tissue at the villous core. Fetal and maternal blood is not normally mixed in the placenta. There is also a paracrine system that links mother and fetus through the placenta. This is an important arrangement for communication between fetus and mother and maternal immunological acceptance of conceptus. ⁶

Adequate fetal growth and subsequent normal birth weight depend on the efficient delivery of nutrients from the mother to the fetus and therefore requires normal uterine perfusion, normal transplacental exchange of nutrients and waste, and normal umbilical blood flow. ⁷

Sonographically, the normal placenta is homogenous and 2 to 4 cm thick, lies against the myometrium and indents into the amniotic sac. The retro placental space is a hypoechoic area that separates the myometrium from the placenta’s basal plate and measures less than 1 to 2 cm. During prenatal sonographic examinations, placental location and relationship to the internal cervical os are recorded. The umbilical cord is also imaged, its fetal and placental insertion sites examined, and its vessels counted. ⁵

The placental thickness is measured trans-abdominally by placing the ultrasound transducer perpendicularly to the plane of the placenta, in the area of the cord insertion at the third trimester, at term the placenta is approximately 3cm thick at the cord insertion. ⁸

An abnormally decreased placental weight has been linked to increased perinatal complications, including intrauterine fetal demise (IUFD) and fetal growth restriction (IUGR). Despite its promise, determining placental weight prenatally using three-dimensional systems is time-consuming and requires expensive technology and expertise. Placental weight can be accurately predicted by two-dimensional ultrasound with volumetric calculations. This method is simple, rapid, and accurate, making it practical for routine prenatal care, as well as for high-risk cases with decreased fetal movement and IUGR. Routine estimated placental volume (EPV) surveillance may decrease the rates of perinatal complications and unexpected IUFD.⁷

This study was conducted to assess the role of measurement of placental thickness and diameter in the third trimester using two-dimensional ultrasound in the determination of low birth weight.

PATIENTS AND METHODS

Type of the study: Observational prospective study was conducted between 15 October 2019 and 10 June 2019. This study was carried out at Sayed Galal University Hospital and Damanhour Teaching Hospital on "400" uncomplicated pregnant women who attended the outpatient clinic at Sayed Galal University Hospital and Damanhour Teaching Hospital, two-dimensional ultrasound measurement of placental thickness and diameter was done to assess its role in the determination of low birth weight.

Duration of follow up: measurement of placental thickness and diameter maximally one week before delivery and the 1^st weight of the baby was measured during the first hour of life to assess the role of measurement placental thickness and diameter in the determination of low birth weight 

Inclusion criteria: Patients aged between 20 and 30 years, singleton viable pregnancies and gestational age from 27 weeks till the time of delivery.

Exclusion criteria: Abnormally situated placenta and placental anomalies and patients with pregestational and gestational diabetes mellitus.

Collecting data:

Every patient eligible for the study is submitted for: Fully informed verbal and written consent, complete detailed medical and obstetric history recorded as a standard paper form and including patient age, height, weight, parity, date of last normal menstrual period, medical disorders and surgical intervention.

Intervention:

Every patient eligible for the study is submitted for a detailed ultrasound examination by two-dimensional ultrasound to determine. Fetal gestational age at time examination by measuring biparietal diameter, femur length, and abdominal circumference.  Expected fetal weight (EFW) by Hadlock's formula (abdominal circumference, biparietal diameter, and femur length). The placental site, diameters, and thickness, and ultrasound scan was repeated every two weeks till delivery.

Outcome data:

Primary outcome: Birth weight is detected within the first hour of delivery.

Secondary outcome: Neonatal status and morbidity including APGAR score, neonatal distress or neonatal death and admission to the neonatal intensive care unit (NICU). Gestational age at the time of delivery and fetal congenital anomalies.

Statistical analysis:

Recorded data were analyzed using the statistical package for social sciences, version 20.0 (SPSS Inc., Chicago, Illinois, USA). Quantitative data were expressed as mean± standard deviation (SD). Qualitative data were expressed as frequency and percentage.

The following tests were done: Independent-samples t-test of significance was used when comparing two means. Chi-square (x²) test of significance was used to compare proportions between two qualitative parameters. The confidence interval was set to 95% and the margin of error accepted was set to 5%. The p-value was considered significant as the following: Probability (P-value); P-value 0.05 was considered insignificant.

RESULTS

Table 1: Maternal age, parity, and mode of delivery among the studied cases

Table 1 shows maternal age, parity, and mode of delivery among the studied cases. Primigravida parity was 152(38%), Multigravida parity was 248(62%), delivery with VD was 211(52.8%) and delivery with CS was 189(47.2%). Age ranged from 20-30 with a mean value of 25.7±2.3.

Table 2: Fetal gestational age (GA) and sex among the studied cases.

Table 2 shows that fetal gestational age (GA) and sex among the studied cases. Males were 202(50.5%) and females were 198(49.5%). The most frequent increase was in week 39 with 151(37.8%) followed by week 38 with 113(28.3%). GA weeks ranged from 27-40 with a mean value of 38.3±2.1.

Table 3: Correlation between neonatal weight and other variables

Total=400. Pearson correlation. *Significant

Table 3 shows that there were significant positive correlations between neonatal weight and GA, Placental diameter, Placental thickness, Biparietal diameter, Femur length & Abdominal circumference (P < 0.05), while there was no statistically significant difference regarding maternal age (P > 0.05).

Table 4: Comparison according to parity regarding fetal and placental measures

^Independent t-test.

Table 4 shows that there was no statistically significant difference according to parity regarding fetal and placental measures.

Table 5: Comparison according to fetal sex regarding fetal and placental measures

^Independent t-test. *Significant

Table 5 shows that there was no statistically significant difference according to fetal sex regarding placental measures (P > 0.05). There was a statistically significant difference according to Biparietal diameter, femur length, abdominal circumference & Neonatal weight (P< 0.05). Fetal measures were significantly higher among males.

Table 6: Comparison according to neonatal weight regarding different variables

^Independent t-test. #Chi square test. *Significant

Table 6 shows that cases with low birth weight significantly had lower Fetal GA, Placental thickness, Placental diameter, Biparietal diameter, Femur length, and abdominal circumference as well as significantly less frequent males.

Table 7: Regression model for the prediction of neonatal weight among the studied cases

β: Regression coefficient, SE: Standard error, CI: Confidence interval, *significant, R²: Coefficient of determination.

Table 7 shows a regression model for the prediction of neonatal weight among the studied cases. GA, placental thickness and male sex were significant factors that increase weight. The following model can explain 75.2% of weight variability: Weight (kg) = -5.746+ 0.209* GA (week) + 0.022* Placental thickness (mm)+ 0.164 (if male). This model is for gestational age from week-27 to week-40.