| What Is A Third Trimester Ultrasound Scan?
It is an ultrasound scan carried out in the third trimester of pregnancy (weeks 28 to 40). Generally most women are only offered 2 scans, one in the first trimester and another called an anatomy scan in the second trimester (although you may be offered an additional scan at week 12 called a nuchal scan). A third trimester scan may be advised where there is some concern over the baby or mother.
Why Is It Done?
Common reasons for performing a third trimester scan include:-
The diagnosis of malpresentation (baby lying in an awkward or dangerous position before birth) is usually easily spotted by a simple clinical examination (no scan necessary). However, in an obese mother the detection of a breech presentation, for example, may be difficult and an ultrasound scan provides a quick and reliable way of resolving the problem.
Here again clinical examination may be difficult and if the patient has not had a scan earlier in the pregnancy the diagnosis can readily be made by ultrasound scanning, which will also reveal the lie and presentation of the fetuses. It will also help identify any umbilical cord problems - such as where a cord is wrapped around one baby's neck.
The placenta is readily seen with ultrasound and the placental site can be identified with considerable accuracy, which is very valuable in dealing with suspected placenta previa. Caution needs to be exercised, however, because there are certain inherent difficulties in precise localization. Placentas found to be low lying in earlier scans often change as pregnancy advances so that the placenta moves upwards. For this reason, placentae which appear to be low-lying on scanning in the second trimester are rarely found to be so when the patient is rescanned in the third trimester. However, in spite of these difficulties an ultrasound scan is extremely valuable in determining the cause of an antepartum hemorrhage (bleeding) in the third trimester (antepartum means it occurs not long before childbirth). The ultrasound findings coupled with the clinical findings allow the diagnosis of placenta previa to be made with confidence and clinical management undertaken accordingly. The detection of placental abruption causing antepartum hemorrhage is more difficult but a large blood clot may be identified. In such cases, however, the clinical diagnosis is usually clear.
Placenta Size And Shape
Some abnormalities in the size or shape of the placenta (such as placental insufficiency) can be detected by ultrasound, for example the large placenta found in women with badly controlled diabetes and the hydropic placenta associated with fetal hydrops (swelling in the fetus due to accumulation of fluids). The size of the fetus is related to the size of the placenta; a small placenta is characteristic of intrauterine growth restriction (IUGR).
Assessing The Baby’s Growth
The pattern of normal human growth while in the womb is not completely understood, but for practical clinical purposes the important group of fetuses are those who show IUGR, a process that results in the birth of a baby whose weight is below the 10th centile for gestational age. These small-for-gestational-age babies are major contributors to perinatal mortality (death from 22 weeks gestation to 7 days after birth), which is 10 times more frequent in growth-retarded fetuses. Growth restriction arises either from poor general nutrition of the mother or because the maternal blood supply of the placenta is reduced. General nutritional deprivation causes symmetrical growth restriction and the birth of a baby who is small all over. Poor placental perfusion, classically associated with gestational hypertension, causes asymmetrical growth restriction. This is where the head grows faster than the body. Asymmetrical growth restriction is due to preferential shunting of blood to the fetal brain, which develops at the expense of the fetal body. When born these babies have small bodies but relatively normal sized heads. Both types of growth restriction are likely to be associated with a high incidence of perinatal asphyxia and neonatal hypoglycemia but the fetuses in the asymmetrical group are the more severely affected. The clinical diagnosis of IUGR is not very accurate, but ultrasound measurements of the diameter and the circumference of the fetal head and the circumference of the fetal abdomen at the level of the liver give a much better assessment of fetal size. By comparing the fetal head and abdominal circumferences (the head-abdomen circumference ratio) the diagnosis of asymmetrical growth restriction can also be made. Because the fetal abdominal circumference bears a reasonably constant relationship to fetal weight, this measurement alone provides the clinician with a good estimate of what the baby would weigh if delivered immediately at the time the scan was performed. In clinical practice, once IUGR has been diagnosed a series of scans at two-weekly intervals is performed until it is clear that fetal growth is being seriously impaired and that delivery should be undertaken. The stage of the pregnancy at which this is done will depend upon individual circumstances and on observations of fetal activity and variations in the fetal heart rate pattern. Diminishing fetal activity is an indication for labor induction of a fetus showing growth restriction.
At the other end of the fetal growth scale, developing macrosomy causing a large-for-gestational-age baby may be seen, for example in pregnancy complicated by gestational diabetes. Here the head-abdomen circumference ratio is reversed, the abdomen growing at a faster rate than the head as a result of disproportionate increase in the size of the fetal liver and body. It is clear, therefore, that by intelligent use of ultrasound and deviations from the normal pattern of fetal growth some of the associated potential disasters can be avoided.
• For more general advice see: pregnancy ultrasound scans See also: Third pregnancy trimester.
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