Correlation of placental thickness with gestational age in second and third trimester using ultrasonography

The accuracy of currently established parameters in estimating gestational age decreases as pregnancy advances. With advancing gestational age, there is an expected linear increase in placental thickness. Placental thickness may thus be used to reliably predict gestational age, especially in later stages. Our study aimed to determine the relationship between placental thickness and gestational age and routinely used fetal growth parameters in the second and third trimesters. Additionally, we aimed to see if the placental thickness was lower in small for gestational fetuses. This study was performed at a tertiary care center and was a prospective observational study. We recruited consecutive 200 pregnant women fulfilling inclusion and exclusion criteria. Using routine antenatal ultrasonography, we obtained several measurements of placental thickness, BPD, HC, FL, and AC. The placental thickness was measured at the level of umbilical cord insertion. The gestational age was calculated using last menstrual period. There was a strong correlation between placental thickness and gestational age (p < 0.001), BPD (p < 0. 001), HC (p < 0. 001), FL (p < 0. 001), and AC (p < 0.001) in both trimesters combined. In the second trimester, there was a strong correlation between placental thickness and gestational age (p < 0.001), BPD (p < 0.01), HC (p < 0.001), and AC (p < 0.001). In the third trimester, there was a strong correlation between placental thickness and gestational age (p < 0.001), BPD (p < 0.001), HC (p < 0.001), FL (p < 0.001) and AC (p < 0. 001). Patients delivering Small for gestational age (SGA) babies had significantly thinner placentas as compared to those with normal-weight babies (p < 0.001). Placental thickness has a strong correlation with gestation age as well as BPD, HC, and AC in the second and third trimesters. Placental thickness is significantly lower in small for gestational age babies. Routine measurement of antenatal placental thickness using ultrasound can help predict gestational age and detect small for gestational age babies.


Background
Currently, first-trimester USG in the embryo or fetus is the most accurate method for estimation or confirmation of gestational age (GA) [1].Accurate estimation of GA is the basis of current routine antenatal USG scanning and is crucial for successful prenatal delivery and optimal postnatal care, especially in developing countries [2].It is imperative for prompt postnatal maternal and neonatal management.Biometric parameters namely biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) are now routinely used in the second and third trimester dating scans [3].However, these parameters can sometimes generate conflicting values of gestational age, for instance in fetuses in breech presentation [4].Moreover, each of these comes with its set of drawbacks [5].One of the many pitfalls reported is the decreased accuracy of these parameters as the gestational age advances [6].This warrants the need to explore additional parameters that could complement the established biometric parameters, particularly in the late third trimester [7].
Placenta plays a critical role in normal fetal growth.It is a multifunctional organ, that performs the critical function of mediating the passage of materials and nutrients between the maternal and fetal circulation.Placental thickness (PT) changes are indicative of normal growth of the "fetoplacental unit" and can be measured by ultrasonography [8,9].Previous literature suggests that there is a linear increase in placental thickness with GA through the course of normal pregnancy [10].
Immature placentas reflect insufficient uteroplacental blood flow.SGA is listed as one of the major causes of perinatal mortality due to neonatal asphyxia, developmental disorders, and other complications [11].There have been conflicting results regarding the association of placental thickness with SGA with no consensus.
Our study aimed to correlate placental thickness (PT) with gestational age (GA), biparietal diameter (BPD), head circumference (HC), femur length (FL), and abdominal circumference (AC) using ultrasonography, in the second and third-trimester pregnancies.Additionally, we aimed to correlate placental thickness with expected fetal weight in predicting small for gestational age infants in our population.

Study design
This was a Prospective observational study, done in a tertiary care center in XXX, from the period of January 2019 to July 2020.The study was conducted after getting clearance from the institutional ethical committee.A written informed consent was obtained from all patients to be a part of the study.200 pregnant women in the second and third trimesters of gestation, presenting to the Obstetrics OPD for routine antenatal checkups, and in-patients referred from the Department of Obstetrics were included in the study.
The inclusion criteria were all pregnant women with gestational age between 15 and 39 weeks with normal singleton pregnancies.The exclusion criteria were maternal co-morbidities like diabetes mellitus, hypertension, severe anemia, heart disorders, jaundice complicating pregnancy and kidney disorders, history of previous IUGR, and adverse fetal outcome.
Patients with placental abnormalities like-placenta previa, coexistent placental pathology, uterine or adnexal mass.Fetal Disorders like intrauterine growth restriction, fetal anomalies, or hydrops fetalis, multiple fetuses, polyhydramnios, and oligohydramnios were excluded from the study.
All the patients were evaluated by ultrasound for routine obstetric scans using GE LOGIQ P9.
MACHINE (Convex transducer 3-5 MHz).The placental thickness was measured in 'mm' at the level of umbilical cord insertion.Placenta was localized in a longitudinal section.Placental thickness measurement was taken from the edge of the echogenic-appearing chorionic plate to the interface of the placenta with the myometrium (Fig. 1).This was done at the level of cord insertion.Precautions were taken for proper identification of the cord insertion site; exclusion of the myometrium and sub-placental veins and measurements were taken during the relaxation phase.Routine BPD, HC, FL, and AC estimation was done (Fig. 2).A composite average of gestational age was derived.All fetuses were followed up till birth.Fetal birth weight and small for gestational age were recorded (Fig. 3).

Statistical analysis
Descriptive analysis was carried out by mean and standard deviation for quantitative variables, frequency, and proportion for categorical variables.Independent sample t-test was used to assess statistical significance.The association between explanatory variables and categorical outcomes The mean placental thickness for each gestational age in the 2nd trimester was derived.Maximum mean placental thickness in the second trimester was found to be 3.07 ± 0.18 at 26 weeks of gestation.Minimum mean placental thickness in the second trimester was 2.01 ± 0.23 at 18 weeks of gestation (Table 1).
The mean placental thickness for each gestational age in the third trimester was derived.Maximum mean placental thickness was found to be 3.45 ± 0.27 cm at 31 weeks of gestation.Minimum mean placental thickness was found to be 2.87 ± 0.29 at 27 weeks of gestation (Table 2).
To find the correlation between placental thickness and other parameters like GA, FL, BPD, HC, and AC, the correlation coefficient was calculated (Table 3).There was a strong correlation between placental thickness and gestational age in the second trimester, with a Pearson coefficient (r) value of 0.559 and a p value of < 0.001.No statistically significant correlation is found between placental thickness and the femur length in second trimester, with r value of 0.125 and p value of 0.333.Placental thickness with BPD and HC both shows statistically significant correlation with r value of 0.550, 0.533 and p value of < 0.001, respectively.
A strong correlation was also found in the second trimester between the placental thickness and AC with r value 0.839 and p value < 0.001 (Table 3).
There was correlation between the placental thickness and the gestational age in the third trimester, with r value of 0.383 and p value of < 0.001.Statistically significant correlation is found between placental thickness and femur length in the third trimester, with a r value of 0.434 and a p value of < 0.001.Placental thickness with BPD and HC both shows statistically significant correlation with r value of 0.438, 0.307, respectively, and p value of < 0.001 and < 0.01, respectively (Table 3).
Placental thickness in the SGA group (mean 2.64 + 0.51) was significantly lower as compared to the normal group (mean 3.02 + 0.57) with a p value of < 0.001 (Table 4).

Discussion
Placental thickness is indicative of normal fetoplacental growth.As gestational age progresses placental thickness increases [12].Thus, measuring placental thickness can help estimate gestational age.Additionally, it may help identify fetal abnormalities.The aim of our study was to correlate placental thickness (PT) with gestational age (GA), biparietal diameter (BPD), head circumference (HC), femur length (FL), and abdominal circumference (AC) using ultrasonography, in the second and third-trimester pregnancies.Additionally, we aimed to correlate placental thickness with expected fetal weight in predicting small for gestational age infants in our population.We found that placental thickness correlated with gestational age in both the second and third trimesters.We found that the placental thickness in the SGA group was lower as compared to the normal group.
We found that there was an increase in the thickness of the placenta as gestational age advances up to almost 35 weeks.From there, a minimal decrease in placental thickness was noticed from 36 to 39 weeks.Conversely, Mital et al. [13] found a steady increase in placental thickness up to 39 weeks.Jain et al. [14] like our findings, found a steady increase in placental thickness up to 28 weeks.Between 15 and 21 weeks of gestation, we found the placental thickness was higher than gestational age in weeks by 1 to 4 mm.Our findings were similar to those of Tiwari et al. [10] who showed that PT was higher than the gestational age by 1-4 mm up to 21 weeks of gestation.Ganjoo et al. [15] stated that the placental thickness was higher by 1.25 mm at 18 weeks and 2 mm at 19 weeks.
In our study, it was seen that there was a significant correlation between placental thickness and Gestational age, BPD, HC, AC, and FL.Karthikeyan et al. [16] in their study too found that there was a positive correlation between placental thickness and gestational age, BPD, HC, AC, and FL.Karthikeyan et al. [16] in their study state that since PT and GA have a linear relationship, PT can be used to substitute abnormal fetal growth parameters.For instance, they stated that in a fetus with hydrocephalus a subnormal BPD could be substituted with PT in addition to the other routine fetal growth parameters to estimate GA accurately.Since our study also showed a linear relationship between PT and GA, our results support the routine use of PT measurement on USG for accurate GA estimation.Placental thickness measurement can prove to be useful in cases with difficulty in assessing measurements like BPD and HC, like in large babies and late in the gestational period.
Studies have earlier shown that low placental thickness can predict low birth weight.Schwartz et al. [17] showed that the mean placental thickness and diameter taken in mid-gestation were significantly less in SGA babies.In our study, we showed that placental thickness in mid and late gestation was significantly less in SGA babies.Thus, the placental thickness can be important until late in the third trimester in predicting SGA babies.There were some limitations in our study.Firstly, serial measurements of placental thickness were not taken.Secondly, all measurements were taken by a single observer.A larger sample size including more patients in each week of pregnancy can produce reference values that can be used in our routine practice.

Conclusions
Placental thickness has a strong correlation with gestation age as well as BPD, HC, and AC in the second and third trimesters.Placental thickness is significantly lower in small for gestational age babies.Routine measurement of placental thickness can help predict gestational age and detect small for gestational babies.

Fig. 2 Fig. 3
Fig. 2 Fetal biometry measurement on USG with gestational age corresponding to 33 weeks 3 days: A HC, B BPD, C FL, D AC (E) PT

Table 4
Comparison of expected fetal weight (in kg) and placental thickness between small for gestational age and normal babies