SONOGRAPHIC CORRELATION OF FETAL NECK CIRCUMFERENCE AND AREA WITH GESTATIONAL AGE AMONG PREGNANT WOMEN IN PORTHARCOURT, NIGERIA.

CHAPTER ONE
INTRODUCTION
Background of the study: The neck represents anatomically a small part of the human body between the head and the thorax. Due to its unique position, it conveys vital structures that communicate between the head and other parts of the human body. It contains conduit of respiration, deglutition and blood to and from the brain as well as important endocrine and neural structures. The structural development of the neck occurs between the 3rd and 8th weeks of gestation. According to Johnston (1990), the five pairs of brachial arches corresponding to the primitive vertebral gill bars that form on either sides of the pharyngeal foregut on the day 22 are the embryologic basis of all the differentiated structures of the head and neck. The gestational age refers to the length of pregnancy after the first day of the last menstrual period. The importance of accurate gestational age in pregnancy cannot be over emphasized. According to Peek et al (1994), the estimation of pregnancy date is important to the mother who wants to know when to expect the birth of her baby, and for her health care providers, so they may choose the times to perform various screening test and assessments. Accurate pregnancy dating assists obstetricians in appropriately counseling women who are at risk of preterm delivery about the likely neonatal outcomes. According to Hall and Carr-Hill (1985), uncertain gestational age has been associated with adverse pregnancy outcome including low birth weight, spontaneous preterm delivery and perinatal mortality independence of maternal characteristics. Pregnancies erroneously thought to be preterm may lead to prolong hospitalization and risk of dangerous medication including tocolytic therapy. It is also essential in the evaluation of fetal growth and detection of intrauterine growth retardation. Gottlieb and Galan (2008) asserts that accurate gestational dating is one of the most important assessment a health care provider make in pregnancy, giving that all of the various management strategies are dependent on knowing where the patient is in gestation. It helps in scheduling invasive diagnostic tests such as chorionic villus sampling or amniocentesis as appropriate timing can influence the safety of the procedure. Also correct biomedical serum screening interpretation depends on the accuracy of dating of pregnancy. Accurate pregnancy dating also
helps in the counseling of patient regarding the option of pregnancy termination.

Different parameters have been employed in the calculation of the gestational age of fetus and these includes-Ovulation date: Most women who know when they ovulate based on formal
methods and record keeping such as daily temperature check and on physical symptoms such as pain upon ovulation or observation of changes in vaginal mucus can use it in calculation of their due dates. It is important to note that this method is not without a flaw as variations in the ovulation period among women complicates the matter. Walker et al (1988) supported this claim when he evaluated 75 ovulation cycles using luteinizing hormone level as a biochemical marker and found that ovulation occurs within a range of 8-31 days after the last menstrual period. Wilcox et al (1993) also stated that the time of ovulation varies greatly in relation to the menstrual cycle, both from cycle to cycle and individual to individual.
Conception date: Calculating expected date of delivery based on conception date is normally used when the conception is medically managed and supervised through technique such as artificial insemination, although this involves an invasive technique.

Last Menstrual Period (LMP): Expected date of delivery is typically calculated based on the date the last menstrual period began according to the mother’s report. This forms the basis of Franz Naegele’s rule which states that the standard definition of gestational term is 280 days from the date of conception to the date of baby’s birth, or 40 weeks from the first day of the
mother’s last menstrual period (Durham, 2002). This definition is on the assumption that the mother ovulates on the 14th day of the 28 days menstrual cycle. He, therefore use this formula to calculate the expected date of delivery. LMP + 7 days – 3 months = Expected date of delivery. Durham (2002) also made us to understand that the observation made by Franz Naegele was not based on empirical data. Calculation of gestational age using last menstrual period has its own shortcomings. This is because of varying length of follicular phase and the fact that many women do not have regular menstrual cycle. In a study by Kramer et al (1988) on the validity of gestational age estimation by menstrual dating in term, preterm and post term found that out of 11,000 pregnant women who underwent early ultrasound, one fourth of all the infant who would have been classified as preterm and one eight of all the infant who would have been classified as post term by menstrual history would have been misdiagnosed. Similarly Campbell et al (1985) demonstrated that of more than 4,000 pregnant women, 45% were not certain about their LMP as a result of poor recall of date, irregular cycles, bleeding in early pregnancy or oral contraceptive use within two months of conception. Geirsson and Busby-Earle (1991) also stated that certain last menstrual period date may not provide a reliable estimate of gestational age. They found out that10-45% of pregnant women cannot provide useful information about their LMP; and 18% with certain menstrual dates have significant difference between menstrual and ultrasound dating.

Fundal Height Measurement: Measurement of symphysis- fundal height which spans from the pubic symphysis to the fundal part of the uterus was used to estimate the gestational age of a fetus. This measurement although, may be a useful adjunct, it does not provide reliable guide as a sole predictor of gestational age. Gardosi and Frances (1999) found that the measurement of fundal height is influenced by the amniotic fluid volume, placenta thickness, myometrial wall thickness, maternal abdominal wall fat and the relationship of the uterus with the bony pelvis.

Quickening: This is the first feel of fetal movement as noticed by the mother. It occurs from about 16-20 weeks of gestational age and has been used as a rough estimation of fetal age.
Also the detection of fetal tone using stethoscope at 18-20 weeks of gestational age is also employed in the estimation of fetal gestational age. Also, this can be useful adjunct, but carries potential error when used as a sole predictor of gestational age.

Ultrasound Scan: The use of ultrasonography has played a vital role in the estimation of fetal gestational age. Kalish and Chervenak (2002) stated that ultrasound assessment of gestational age has become an integral part of obstetric practice in recent times. Currently, the sonographic estimation of gestational age is derived from calculation based on fetal measurement and serves as an indirect indication of gestational age. Hadlock et al (1984) states that numerous equations regarding the relationship between fetal biometric parameters and gestational age has been described and has proved early antenatal ultrasound to be an objective and accurate means of establishing gestational age. The biometric parameters used in the estimation of fetal gestational age are as follows:-
Gestational Sac Diameter (GSD): The gestational sac is the earliest sign of pregnancy in ultrasound. The gestational sac is a fluid-like sac surrounded by an echogenic rim- the developing chorionic villi within the endometrial cavity. According to Crispigny et al (1988), the gestational sac is visualized as early as five menstrual weeks using transvaginal scan. Three measurements are made which includes the long axis, anterior-posterior diameter and transverse diameter. The average of these three measurements are calculated and then used in the estimation of fetal gestational age.

Crown Rump Length (CRL): This is the measurement of the fetal length from the tip of the cephalic pole to the tip of caudal pole. Callen (2000) advised that three adequate CRL measurements should be taken and the average used in the determination of gestational age. The accuracy of the crown rump length has been well documented in the medical literatures.
Specifically, gestational age can be estimated safely with a maximal error of 3-5 days in the first trimester using crown rump length according to Kalish et al (2004) and Wiser et al (1994). Chaudhuri et al (2013) noted that in twin pregnancies, the crown rump length of the smaller fetus is more accurate in determining gestational age.

Biparietal Diameter (BPD): The BPD is imaged in the transaxial plane of the fetal head at the level of thalami in the midline, equidistance from the temporoparietal bones and usually the calvum septum pelucidium anteriorly. The BPD was the first fetal parameter to be utilized in the determination of gestational age in the second trimester before more recent studies have evaluated the use of several other biometric parameters. Gestational age estimation using a single biparietal diameter has an accuracy of ± 10-11 days in the second trimester although, recent study by Wu et al ( 2012) found that biparietal diameter share similar accuracy with CRL in late first trimester ultrasound estimation with additional advantage of lower random error.
Femur Length (FL): The femur length measurement is taken along the long axis of the bone. A straight measurement of the osseous portion is taken from one end to the other end disregarding bone curvature. The accuracy of FL and BPD are similar in the third trimester although there is controversy regarding the accuracy of FL prior to 26 weeks of gestation. Mongeli et al (2003) stated that in late second trimester, the femur length can be used and is nearly as accurate as head circumference and biparietal diameters.

Abdominal Circumference (AC): The abdominal circumference is obtained in the transaxial view of the abdomen at the level of the fetal liver, using the umbilical portion of the left portal vein as the landmark. The fetal stomach is at the same level which is slightly caudad to the fetal heart and cephalad to the kidneys. The abdominal circumference is most useful in the
determination of fetal weight. The accuracy of abdominal circumference in the estimation of gestational age is less than all other predictors of gestational age at term (Benson and Doubilet, 1991).
Apart from these traditional predictors of gestational age, there are other ancillary biometric parameters used in the estimation of fetal gestational age. These include- ear size, orbital diameter, cerebral diameter, placenta thickness, fetal neck circumference, fetal neck area and foot length. Benson and Doubilet (1991) noted that the accuracy of all the traditional predictors of gestational ages worsen progressively as pregnancy advances to the third trimester. These show that none of these parameters can be reliably used to estimate gestational age in third trimester as there are gross variations in growth from one fetus to another. It is due to these significant margins of error in ultrasound measurement that some school of thought advised that gestational age calculation should not be changed from that calculated from the last menstrual period unless the discrepancy is more than two weeks. Also, as a result of these variations, Gottlieb et al, (2008), observed that in addition to the traditional biometry, ancillary biometric and non biometric measurement can help narrow the biological variability between fetuses.

Fetal neck circumference and area seems to be good ancillary biometric parameters which are easier to identify and measures. Hata et al (1988) and David et al (2007) found a linear increase in fetal neck circumference and area with increase in gestational age. There was discrepancy noted in the two studies from 32 weeks of gestation upwards where the measurements from David and colleagues become higher than that of Hata and colleagues. This discrepancy was attributed to either morphological difference in the population studied or enhancement in the sonographic resolution of the ultrasound machine used in recent times compared to that used two decades ago. Mittendorf et al (1990) also observed that gestational age of an average healthy, white, private care, and primiparous Irish American woman average 288 days from LMP to birth; 8 days longer than that of Franz Naegele’s rule. They further concluded that ethnicity among other factors affects the gestational age of fetus. Degan (2001) also supported this idea when he stated that various epidemiological factors involved in the fetal growth should be considered and specific chart for different communities should be used when possible. This may be the reason why Kurtz (2007) stated that there were many well established charts that has been in use for a long time; however, marked difference between population sometimes forces researches to build nomograms for different race, hence the need for this study. This study is therefore aimed at creating a reference range nomograms of the axial neck circumference and area in second and third trimesters in a Nigerian population.

Statement of Problem
1. There seems to be racial difference in fetal neck circumference and area measurements in previous literatures (David et al 2007), thus the need to have an indigenous nomogram.

Research Questions
1. Can fetal neck circumference and area be used as accurate predictor of gestational age in second and third trimester?
2. Is there any geographic variation in the fetal neck circumference and area between Blacks (Nigerian) and the Caucasians (USA)?

Objectives of the study
1.41 General Objective
1. To measure fetal neck circumference and area as sonographic indices for determining gestational age in second and third trimesters.

Specific Objectives of the Study
To establish nomograms of fetal neck circumference and area for determining gestational age in second and third trimesters in a Nigerian population.
To compare the results of this study with that obtained from the Caucasians (USA).

Significance of the Study
Fetal neck circumference and area nomograms created may be used an indigenous biometric parameters for the estimation of fetal gestational age in second and third trimesters. Fetal anomaly associated with increase or decrease in the fetal neck circumference and area can be identified when measuring the neck circumference and area.

Scope of Study
This study was conducted at the radiology department of Braithwaite Memorial Specialist Hospital (BMSH) in Portharcourt, Nigeria.

Operational Definition of Terms
Ultrasound: This is a mechanical longitudinal wave with frequency above the range of human hearing which is greater than 20 KHz. It is produced by the oscillatory motion of the particles in a medium creating region of compression and rarefaction (Sanders, 2007).
Transducer: This is a device which converts electrical energy to mechanical energy and vice versa. It comes in different frequencies- typically 2.5, 3.5, 5, 7 and 10 MHz with
decrease in penetration as frequency increases. Transducers can come in many formats which includes- linear array, vector, sector and curved array and can also be classified based on the purpose such as transabdominal, transvaginal, transoesophageal, transluminal and transcardiac transducers.

Brightness Mode:
This is one of the ultrasound image displays. Here the amplitude modulated signals are converted into dots, which vary in brightness depending on the strength of the returning echo.

Gestational age: The gestational age is the period of time from the day of conception to the day of delivery. The gestational age is estimated to be 40 weeks from the date of last
menstrual period to the time of delivery.

Trimesters: Trimesters means three months. Pregnancy is subdivided into three trimesters. The first trimester starts from the first month to the third months of the pregnancy while second trimester starts from the fourth month to the sixth month. The third trimester starts from the seventh month to the ninth month.

Fetal Neck: The fetal neck is the part of the body that connects the head to the thorax. It serves as a conduit to several organs and contains muscles, nerves, vascular
bundles and the cervical spines.