SONOGRAPHIC ESTIMATION OF GESTATIONAL AGE BY PLACENTAL THICKNESS AMONG PREGNANT WOMEN IN ENUGU, NIGERIA

INTRODUCTION

Background of the Study: The placenta is an organ that connects the developing fetus to the mother. The placenta performs respiratory, excretory, nutritional and endocrine functions for the fetus. It transfers gases, such as oxygen and carbondioxide, waste products like urea, nutrients like glucose and hormones between the maternal and fetal circulation (Kelvin, 1994). In addition, the placenta protects the fetus from immune attack by the mother and induces maternal blood flow to the placenta. A portion of the placenta membrane called the placental barrier protects the fetus by prohibiting some harmful micro organism from entering fetal circulation (Betlye, 2008).

The placenta has both fetal and maternal portions. The fetal portion of the placenta consists of villi of the chorion frondosum. A branch of an umbilical artery enters each villus and ends in a capillary plexus from which the blood is drained by a tributary of the umbilical vein. The maternal portion of the placenta is formed by the decidua placentalis containing the intervillous space. The uterine arteries and veins pass to and from the intervillous space through the basal plate and the stratum spongiosum and the boundary layer. The fetal and maternal blood currents traverse the placenta, the former passing through the blood vessels of the placental villi and the latter through the intervillous space. The two currents do not intermingle, being separated from each other by the delicate walls of the villi. Nevertheless, the fetal blood is able to absorb, through the walls of the villi, oxygen and nutritive materials from the maternal blood, and give up to the latter its waste products. The blood, so purified, is carried back to the fetus by the umbilical vein (Sadler, 2004). The placenta develops at the implantation site at about the fifth week of gestation and by the ninth or tenth week the diffuse echo texture of the placenta is clearly apparent at sonography. The fully formed placenta is approximately 2.3cm in thickness at the insertion of the placenta and the average weight at birth is 500g (Kelvin, 1994).

Determination of gestational age is important because it gives an idea about the fetal growth and development during pregnancy and whether a fetus can live outside the uterus. The estimation of pregnancy dates is important for the mother, who wants to know when to expect the birth of her baby, and for her health care providers, so that they may choose the times at which to perform various screening tests and assessments, such as serum screening, assessment of maturity, and induction of labour for postdates pregnancies (Max, 2010). Determination of gestational age is also important because it provides valuable information regarding expected or potential problems and directly affects the medical treatment plan for the baby (Neil, 2009). The events of prenatal development usually occur at specific gestational ages. The gestational timing of a toxin exposure or infection can be used to predict the potential consequences to the fetus. From the foregoing; emphasis is therefore laid on gestational age assessment. Gestational age was initially estimated using only the woman’s last menstrual period (LMP) and clinical methods like uterine size assessment, time of quickening/initial perception of fetal movement and fundal height. These methods have many limitations; for instance, in using LMP to estimate gestational age (GA), irregular menstrual periods or women who cannot remember the first day of their last menstrual period and normal variation from the average ovulation date pose limitations. Fundal height introduces up to 8weeks variation in the second and third trimester GA (Robin, 2007), while initial perception of fetal movement vary greatly among women.

Presently, ultrasonography is seen as an effective way of dating pregnancy (Rubby et al., 1998). Gestational sac was the first ultrasound biometric parameter to be used. Later in 1973, Robins introduced the use of crown rump length (CRL). This was followed by the use of other parameters like head circumference (HC), abdominal circumference (AC), femur length (FL), and biparietal diameter (BPD). CRL is more accurate when used to measure GA in the first trimester. It is most accurate between 7 and 13weeks (Woo, 2006). It has an accuracy range of ± 4days. CRL has a limitation that its accuracy reduces if measurement is taken when the fetus is temporally stretching (straightening) and also as the pregnancy advances due to difference in growth rate. Mean gestational sac diameter is an effective way of measuring GA between 5 and 6 weeks with an accuracy of ± 5 days. This cannot be used as the pregnancy advances (Kariki et al., 2006). Davis et al., (1993) noted that there is a racial and sex dependence in BPD, HC, AC, and FL as black fetuses had a significant longer FL than white fetuses. He also noted that males have longer BPD, HC, and AC than females. Lubsky et al, (2006) noted that male fetuses have a significant longer HC, and larger BPD when compared to females. They also noted in 2007 that fetuses in breech position have a significant lower BPD when compared to HC. Some differences are also observed in fetal abnormalities like low birth weight for example Gianluigi et al., (2002) noted a reduction in FL in dwarfism, while Kyle et al., (2007) found a decrease in AC, and HC and an increase in BPD in autism especially in multiplex autism.

Hence, there is need to use a biometric parameter that may be used to establish fetal anomaly and can also be used to estimate GA. Placenta is very easy to identify sonographically and some studies like Hoddick et al., (1985), Eluchala et al., (2004) and Ohagwu et al., (2008) have shown that there is a linear increase in placental thickness with gestational age. Studies like Anupama et al., (2001) and Mital et al., (2011), conducted in India showed that the maximum placental thickness at 39 weeks is 37.5mm. Another study on placenta thickness done by Yuranga, (2011), showed that at no gestational age was the placental thickness greater than 40.0mm. He concluded that any value above 40.0mm indicates placentomegaly. Benirschike and Kanfmann (1990) noted that placental thickness above 40mm is associated with maternal diabetes mellitus, fetal hydrops, or intrauterine infection. A study carried out in Markudi Nigeria by Ohagwu et al., (2008) showed that the maximum placental thickness at 39 weeks is 45.1mm. A significant difference seems to exist between the report of Ohagwu et al., (2008) and previous literature by Anupama et al., (2001) and Mital et al., (2011), Yuranga, (2011) on the normal maximum placental thickness. Reasons stated for these discrepancies by Ohagwu et al., (2008) include probable thicker placenta in Negros or observer errors. They recommend that similar study should be carried out in the same race.

The reviewed literatures indicate that placental thickness appears to be a good biometric parameter for gestational age estimation hence the need for this study. The aim of this study is to estimate GA from placental thickness in normal singleton gestation in the second and third trimesters for pregnant women in Enugu Nigeria and thus determine if there is a racial dependence in placental thickness values.

Statement of Problem

1. Late detection of fetal anomalies and hence inaccurate estimation of gestational age has been noted in conditions like dwarfism, macrocepalus and microcephalus, using some indices like FL, BPD and HC for estimating gestational age (Gianluigi et al., 2002). Hence there is need to try other indices that may give a better pointer to fetal anomaly.
2. There are marked variations in the accuracy of many biometric parameters used for estimating GA (Kariki et al., 2006). There is need for more accurate methods to determine gestational age especially as pregnancy advances in age.
3. There seems to be a racial difference in placental thickness values in previous literatures (Ohagwu, et al., 2008). Thus the need to embark on this study.

 

Purpose of Study

1. To determine gestational age by the use of placental thickness measurements among pregnant women in Enugu.
2. To establish the normal range of values of placental thickness for various gestational ages in the second and third trimesters.
3. To estimate gestational age in the second and third trimesters using placental thickness as a biometric parameter.
4. To determine if there is a racial dependence in placental thickness values.

 

1. Fetal abnormalities have been associated with a relative increase or decrease in placental thickness (Malati et al., 2010). Therefore knowing the normal range of values of placental thickness for each gestational age will help in early detection of fetal abnormality associated with changes in placental thickness.
2. Placental thickness can be used as a sonographically derived parameter for estimating gestational age when there are doubts that other parameters may give inaccurate gestational age.
3. Placenta is easy to identify sonographically and measurement of its thickness and correlation with gestational age will be ideal in a busy ultrasound clinic.

 

Scope of Study

The study was carried out at university of Nigeria teaching hospital Ituku/Ozalla, Enugu, Nigeria.

Hypothesis

H0: There is no correlation between gestational age and placental thickness.

H1: There is a correlation between gestational age and placental thickness.

Operational Definition of Terms

Placenta: The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination and gas exchange via the mother’s blood supply. The placenta is referred to as the fetal lung. The fully formed placenta is a red discoid structure approximately 2-3cm in thickness at the insertion of the umbilical cord. The average weight at term is 500g2 (Kevin, 2003; Sadler, 2004).

Umbilical Cord: This is also called the birth cord or funiculus umbilicomalis. It is the connecting cord from the developing embryo or fetus to the placenta. It contains two arteries and a vein. The umbilical vein supplies the fetus with oxygenated, nutrient rich blood from the placenta. Conversely, the umbilical arteries return the deoxygenated, nutrient depleted blood. The cord, which is up to 60 cm long, ceases to function after birth and is clamped and cut about 2·5 cm from the infant’s abdominal wall. The stump shrivels and falls off within two weeks, leaving a scar which forms the umbilicus (Sadler, 2004).

Gestational Age: This is the age of the embryo or foetus. It is the time measured from the first day of the woman's last menstrual cycle or from 14 days before conception (fertilization) to the current date. It is measured in weeks and days. A normal pregnancy can range from 38 to 42 weeks (Neil, 2009; John, 2004).

Ultrasonography: This is a radiological procedure that uses high-frequency sound waves greater than 20KHz to scan the body part, creating a picture (sonogram) of the body part imaged (Robin et al., 2007).

Pregnancy: The period from conception to birth when a woman carries a developing foetus in her uterus. Pregnancy is typically broken into 3 periods or trimesters. Childbirth is usually about 38weeks after conception in women who have a menstrual cycle length of 4 weeks. This is approximately 40weeks from the last menstrual period. The world health organization defines term for delivery as between 37 to 42 weeks (Williams, 2007).

Second Trimester: This is referred to as weeks 13-28 of the pregnancy. At this trimester movement of the fetus often referred to as quickening can be felt. The placenta fully functions at this time and the fetus makes insulin and urinates. The reproductive organs can be distinguished. Third Trimester: This is between 26-40 weeks. Here the fetus gains up to 28g per day. The woman’s belly will transform in shape as the belly drops due to the fetus turning in a downwards position ready for birth. The fetus begins to move regularly and is felt by the woman.

Obstetric Ultrasonography: The application of medical ultrasonography to obstetrics in which sonography is used to visualize the embryo or fetus in its mother’s uterus (womb). The procedure is often a standard part of prenatal care, as it yields a variety of information regarding the health of the fetus as well as regarding the progress of the pregnancy.