SONOGRAPHIC DETERMINATION OF GESTATIONAL AGE IN THE SECOND AND THIRD TRIMESTER FROM TRANSVERSE CEREBELLAR DIAMETER AMONG PREGNANT WOMEN

CHAPTER ONE
INTRODUCTION

Background of the Study: Accurate knowledge of fetal gestational age (GA) is important for the attainment of best possible antenatal care and successful pregnancy outcome. Ultrasound is a useful modality for the assessment of gestational age (GA) and this can greatly impact on obstetric management and improve ante partum care (Robin et al., 2009). The routine use
of diagnostic ultrasound in obstetrics may be largely due to its non invasiveness, affordability and absence of ionizing radiation. The cerebellum lies in the posterior cranial fossa. It lies dorsal to the pons and the medulla, separated from them by the 4th ventricle. The cerebellum develops from the dorsolateral part of the alar lamina of the metencephalon. In the embryo, cerebellum appears at the end of the fifth week as well as overriding the 4th ventricle (Nitsana et al., 2004). Measurement and demonstration of fetal cerebellum is useful in assessing
gestational age. A prospective study carried out on 50 antenatal patients (20-40 years of age) between 14-40 weeks of pregnancy attending the clinic for routine ultrasound examination showed that transverse cerebellar diameter is useful in gestational age assessment (Prabhat et al., 2010). Physiologically, the cerebellum assists in the coordination of fine movements, muscle tone, balancing and equilibrium in the body (Richard et al., 2010).

Assessment of gestational age is of great importance in obstetric care, and some pregnancies considered to be overdue are due to inaccurate assessment of gestational age. Some gestational age that are not known has been associated with adverse pregnancy outcomes including low birth weight, prenatal mortality and spontaneous delivery, independent of maternal characteristic (Robin et al., 2009). Accurate gestational age assessment is of great importance in obstetric practice. It helps in counseling patients regarding the option of pregnancy termination, evaluation of fetal growth and detection of intrauterine growth restriction (IUGR). It may also help to avoid undue parental anxiety from miscalculations and superfluous invasive procedures, which may increase pregnancy loss (Kalish and Chervenak, 2002). Determination of fetal gestational age requires the measurement of some parameters such as biparietal diameter (BPD), abdominal circumference (AC), head circumference (HC), femur length (FL) and transverse cerebellar diameter (TCD). This can be achieved by the
use of ultrasonography (Roger and Tom, 2007). After the first trimester, crown rump length (CRL) is not useful in fetal age determination because of fetal posturing and size. Furthermore, a biological variation with advancing gestational age and fetal position makes BPD not to be accurate in fetal age determination in the third trimester. Fetal long bones used in prediction of GA are sometimes technically difficult to measure and may not give accurate gestational age of fetus.

However, previous researcher has demonstrated that TCD can serve as a reliable predictor of GA in the fetus and is a standard against which aberrations in other fetal parameters
can be compared, especially when the GA cannot be determined by the date of last menstrual period or an early pregnancy scan (Mark et al., 2001). The aforementioned common GA estimation parameters may have pitfalls as pregnancy advances in late trimester in our population. Inadequate assessment of gestational age at later stage of pregnancy and inability to determine the fetal gestational age at the presence of intrauterine growth restriction is as a result of not using the correct parameters (Halit et al., 2002). This may have motivated future researchers to explore the accuracy of fetal biometric GA parameters such as the TCD especially in growth restricted fetuses. Interestingly, previous study found out that the TCD of growth-impaired fetus were lower than expected, but still within the normal range (Kazumusa et al., 2001). Human cerebellum is resistant to chronic hypoxemia due to brain sparing phenomenon and in the human fetus, cerebellar growth may be least affected by intrauterine growth retardation (IUGR) (Saifon et al., 2006). Transverse cerebellar diameter showed a good correlation with gestational age (correlation co-efficient = 0.991 and 1 < 0.001) (Prabhat et al., 2010).

Literature reviewed showed paucity of information on the use of transverse cerebellar diameter for dating GA in Nigerian populations. Most studies were done in foreign
countries like Australia, India and Thailand (Saifon et al., 2006). The objectives of this study were, therefore, to determine the TCD normogram in relation to patient’s LMP and
fetal FL, BPD, HC and AC parameters; to establish a relationship between TCD and GA obtained from patient’s LMP, fetal FL, BPD, HC, and AC parameters in a Nigerian
population, and to compare the mean TCD values from present study with that obtained from previous literature.

Statement of the Problem
Assessment of gestational age is of great importance in obstetric care, and some pregnancies considered to be overdue may be due to inaccurate assessment of gestational age. The most frequently used biometric parameters for the estimation of gestational age are the fetal biparietal diameter, femur length, abdominal circumference, head circumference, last menstrual period, quickening and fundal height measurement. However, they are sometimes difficult to measure and may not give the accurate GA of the fetus (Roger and Tom, 2007; Robin et al., 2009). This provides the challenges for more research. Variations in the accuracy of many fetal gestational age estimation parameters are marked in late pregnancy. Thus there is need for more accurate methods of determining gestational age as pregnancy advances in age (Kramer et al., 2006).

1.3 Objectives of the Study
1.3.1 General Objectives
To determine the gestational age in the second and third trimesters from transverse cerebellar diameter among pregnant women in Enugu Metropolis by sonography.
1.3.2 Specific Objectives
1. To determine the transverse cerebellar diameter in relation to patient’s LMP and fetal FL, BPD, HC and AC parameters.
2. To correlate the measured transverse cerebellar diameter with gestational age obtained from last menstrual period and FL, BPD, HC and AC parameters.
3. To establish a relationship between transverse cerebellar diameter and gestational age obtained from patient’s LMP.
4. To compare the mean transverse cerebellar diameter values from present study with that obtained from previous literatures (Nepalese population and Thai
population).

1.4 Significance of the Study
Normal range of TCD for each gestational age (14 – 40 weeks) will help in early detection of intra uterine growth restriction. Anomaly detection at later stage of pregnancy is of practical importance. This will help the health care providers to prepare for the accurate time and best fetal surgical facilities that will be used on fetuses with anomalies such as encephalocele in order to avoid fetal morbidity and mortality.

1.5 Scope of the Study
The study was carried out at Enugu State University Teaching Hospital Parklane, Enugu and it involves pregnant women who came for antenatal care at period of study.

1.6 Operational Definition of Terms
Cerebellum: The portion of the brain that lies behind the fourth ventricle and below the tentorium. It helps in fine movement coordination, muscle tone, balancing and
equilibrium in the body (Richard et al., 2010).
Ultrasound: Is a name given to high frequency sound waves, over 20,000 cycles per second (20KHz). These waves (inaudible to human) can be transmitted in beams and are
used to scan the tissues of the body to produce images. (Takedas et al., 2008).
A transducer (probe): Device capable of converting energy from one form to another. It is also a device which turns an electrical signal into a sound pulse of the same frequency.
Transducer operates on piezoelectric effect (Barys and Andy, 2010).
Sonography: Is a technique which uses high-frequency sound waves in acquisition of images. It uses a probe to send pulses of sound waves into a body in order to construct an
image of a body organ (Dubose and Baker, 2009).
Focusing: Adjustment of ultrasound beams so that it converges at a particular depth in order to improve resolution (Roger and Tom, 2007).
Gestational Age: The period during which embryo develops (about 266 days in humans). The state of being pregnant; the period from conception to birth when a woman
carries a developing fetus in her womb (Neil et al., 2009).