Prenatal diagnosis is defined as the total of screening and diagnostic examinations concerning the fetus and the pregnant woman until birth. This definition encompasses examinations for maternal infections such as toxoplasmosis, rubella, varicella, syphilis and human immunodeficiency virus (HIV) that can have an effect on embryonic and fetal development, as well as routine checks of maternal blood count and blood glucose levels which might have an influence on fetal growth in the late second and third trimesters. It also implies screening for maternal blood group and Rhesus factor to detect a possible risk of incompatibility.
Although these tests might be of great importance, prenatal diagnosis has benefited mainly from the extremely rapid development of ultrasound techniques. Ultrasound's first obstetrical use was for verifying placental localization and fetal heart activity.1,2 In the following years ultrasound could contribute immensely to hitherto unsolvable problems and questions such as exact determination of pregnancy length,3 detection of twin pregnancies4,5 and fetal malformations.6–8 The development of amniocentesis led to a new accuracy in diagnosing genetic disorders9,10 conversely influencing ultrasound examiners to search for specific fetal ominous markers to detect fetuses with chromosomal abnormalities.11 New technical developments, such as Doppler assessment of the blood flow in maternal and fetal vessels, allowed for improved surveillance of fetal well-being, intrauterine growth retardation and pre-eclampsia.12–15
Improved ultrasound resolution had already made it possible to screen for fetal abnormalities in the first trimester of pregnancy, leading to a new understanding of embryonic and fetal development. Ultrasound experts recognized, for example, that all fetuses between 11 and 14 weeks present with a small fluid layer in the neck region.16 This fluid accumulation, called nuchal translucency (NT), is usually increased in fetuses with aneuploidies and specific malformations such as congenital heart defects,17–19 exomphalos,20 congenital diaphragmatic hernia21 and a number of syndromes.22 This particular nuchal translucency is now routinely used as a screening tool for the early detection of Down syndrome fetuses.
These and other impressive detections and innovations are responsible for a revolutionary change in the routine schedule of prenatal examinations. Only 15 years ago the emphasis of prenatal ultrasound examination was placed on weeks 18–22, as the majority of fetal malformations are detectable only at this period of gestation.
The second important time period for ultrasound examinations was between 30 and 36 weeks, mainly for examining fetal growth, placental and fetal position and fetal well-being. Today, however, it has been shown that the most important examination is between 11 and 14 weeks of gestation. This article will try to point out the characteristics and the relative importance of the current examinations.
The time period between 11 and 14 weeks of gestation is used for the measurement of the NT. Fetuses must be between 45 and 85 mm in crown–rump length (CRL) which also allows correcting for the exact gestational age. The exact way of measuring the NT is described in detail on www.fetalmedicine.com. The fetus should be lying in a neutral relaxed position; it should be depicted in an exact midsagittal plane as hyperextension and flexion of the neck would increase or decrease the NT by approximately 0.5 mm. For the exact measurement only the head and upper thorax of the fetus should be visible. The magnification should be as large as possible; the ultrasound machine should have a high resolution and a cineloop function. Slight movements of the callipers should produce a change of only 0.1 mm in the measurement. The maximum thickness between the skin and the soft tissue overlying the cervical spine should be measured. The callipers should be placed on the lines that define the NT thickness. More than one measurement must be taken and the highest one should be recorded.
A large UK multicentre study published in 1998 showed that a combination of maternal age and NT could detect 77% of Down syndrome fetuses at a false-positive rate (FPR) of 5%.23
Although these results outdid the detection rates derived from maternal age alone by far, additional improvement was possible by including free beta human chorionic gonadotrophin (fβhCG) and pregnancy-associated plasma protein A (PAPP-A) as biochemical markers. fβhCG is a glycoprotein normally produced by the placenta which has been shown to be considerably increased in Down syndrome fetuses, whereas PAPP-A, a protease from the placenta, is usually markedly decreased in Down syndrome fetuses. This combination of NT, maternal age, fβhCG and PAPP-A, from now on called the ‘combined test’, led to a detection rate for Down syndrome fetuses of 85–90% at a FPR of 5%.24–28 Another advancement regarding both better detection and lower FPRs was achieved by including additional sonographic markers into the risk calculation. This includes the fetal nasal bone, the ductus venosus flow and the blood flow in the tricuspid valve (Figures 1–3).
An absent nasal bone, for instance, was found in 73% of Down syndrome fetuses but in only 0.5% of normal fetuses. Including the fetal nasal bone as a factor along with maternal age and NT improved the sensitivity for the detection of Down syndrome cases to 85% by decreasing the FPR to about 1%.29 Very similar improvements were made by including reverse ductus venosus flow30 and tricuspid regurgitation31 (Figures 4–6). An innovative first-trimester riskorientated two-step approach, taking maternal age, NT and biochemistry into account, and using additional sonographic markers exclusively between the risk cut-offs 1 in 101 and 1 in 1000, identifies more than 90% of the Down syndrome fetuses at a FPR of 2–3%.32
The search for Down syndrome fetuses is only one in a number of investigative options in the first trimester. Between 43.6% and 64.7% of fetal structural abnormalities can already be detected at that early stage of pregnancy.33,34 A new first-trimester marker, for example, the so-called ‘intracranial translucency’, allows the diagnosis of open spina bifida which until recently could be detected only in the second trimester.35 The risk of severe conditions such as pre-eclampsia and intrauterine growth restriction can also be assessed in the first trimester, using both sonographic and biochemical markers such as uterine blood perfusion, placental volume, placental bed perfusion, PAPP-A, placental growth factor, inhibin-A, pentraxin 3 and so on.36–41 Moreover, recent publications have focused on assessing the risk of other severe and dangerous obstetric conditions such as preterm delivery, fetal macrosomia and the risk of gestational diabetes mellitus.42–47
Another very important aspect of first-trimester ultrasound is the clear-cut differentiation between mono- and dichorionicity in twins. In twin pregnancies chorionicity, rather than zygosity, is the main factor determining pregnancy outcome.48 This differentiation is feasible by examining the placental location and searching for the so-called lambda and T sign49 (Figure 7a and b).
The sheer number of publications that focus on the first trimester emphasizes its unique importance in prenatal diagnosis. We have learned the lesson that we can influence the course of a pregnancy only if the diagnosis is made in good time and any possible therapy started early in pregnancy.
Ultrasound screening in the second trimester is routinely performed between 18 and 22 completed weeks. At this time most of the fetal malformations are principally detectable. If the pregnant woman has had a first-trimester scan, the examination should be focused mainly on the presence or absence of structural malformations. The detection of Down syndrome fetuses is no longer of utmost importance at the second-trimester fetal anomaly scan, as it can be assumed that more than 90% have already been detected in the first trimester. If the woman has not had a first-trimester screening, the focus should still be on the detection of Down syndrome fetuses. At this stage of pregnancy, NT, however, has lost much of its previously outstanding importance; the risk of a Down syndrome fetus can be assessed by searching for so-called soft markers. These are signs indicative of Down syndrome such as the humerus or femur length below the fifth percentile, pyelectasis, nuchal fold thickening, absent or hypoplastic nasal bone, echogenic bowel, short ear length, chorioid plexus cysts, echogenic heart focus and specific structural malformations such as an atrioventricular septum defect of the heart. Studies have shown, however, that the soft-marker screening for Down syndrome performs unsatisfactorily.50–52 Detection rates for most of these markers are low. Only one marker, nuchal fold thickening, may be useful, as its presence increases the risk of Down syndrome by approximately 17-fold.52
The actual task of the second trimester scan is the detection of structural abnormalities. Several basic requirements are necessary to fulfil this job:
The ultrasound machine used should meet high technical standards.
The sonographer should be sufficiently trained in the detection of fetal malformations: this includes theoretical and practical training in a fetal medicine centre for a minimum of 2 years where they can consolidate their knowledge of fetal development, genetics, fetal malformations, diagnostic testing, maternal and fetal physiology, perinatal pathology, etc.53
The examination should be structured by diligently processing a detailed and compulsory list of typical sonographic planes. These planes are most suitable for detecting the majority of relevant malformations.54
All shots that meet the stipulated sonographic planes should be stored electronically.
There are, for example, a minimum of planes necessary to properly examine the fetal heart:
An abdominal vessel view to examine the cardiac situs, the course of the aorta and inferior vena cava in relation to abdominal organs such as the stomach (Figure 8).
The four-chamber view to appreciate the size and typical structure of the chambers, the AV valves, the intraventricular septum, the foramen ovale, the septum primum and secundum, and the pulmonary venous return (Figure 9).
Left ventricular outflow tract mainly to confirm the size and relationship of the ascending aorta and observe the area between the intraventricular septum and the anterior wall of the aorta but also look at the aortic valve movements (Figure 10).
Right ventricular outflow tract, i.e. the three-vessel view to ensure the size and relationship of the pulmonary artery and pulmonary valve (Figure 11).
The tracheal view to compare the size of the ductal and aortic arch and its anatomical relationship (Figure 12).
The flow across each heart connection as seen with Doppler flow mapping.55
The requirements for the examinations of all other organs are comparable, which leads to more than 20 planes to be visualized and electronically stored for one proper fetal anomaly scan. The reason for all these detailed steps of examination is on the one hand to find all possibly detectable fetal malformations and on the other hand to establish high and robust standards which can provide some legal security in case a major malformation is missed.
Apart from examining the fetus for possible malformations, the second-trimester examination can be extended by checking for uterine artery Doppler flow in order to assess a woman's risk of acquiring pre-eclampsia and experiencing fetal growth restriction.14,15 An increased pulsatility index with notching is the best predictor of pre-eclampsia (positive likelihood ratio 7.5 among low-risk patients) but less effective for predicting fetal growth restriction. Nevertheless, these indices can be used in clinical practice, especially in high-risk patients.56
The ultrasound examination in the third trimester is routinely performed between 30 and 36 weeks of gestation. Great importance is attached to fetal and placental position, assessment of fetal growth, amniotic fluid and fetal well-being by performing Doppler studies of the umbilical artery.
The routine use of ultrasound in the third trimester is again of great importance. In the early days it could determine the fetal position. It can, moreover, demonstrate the probability of the spontaneous rotation of a fetus to the vertex position during pregnancy in primi- and multiparous women and thus allow either timely conversion to the vertex position or planning a caesarean delivery.57,58 The determination of the placental position is also of utmost importance after the second and especially in the third trimester. Between 18 and 22 weeks, 2–4% of all placentas reach or overlap the internal cervical os,59 whereas at term only 0.4% of the placentas are low lying.60,61 Another increasingly important focus of placental examination is the prenatal sonographic diagnosis of placenta accreta. This becomes necessary because of the steadily rising incidence of this condition in a time of increasing frequency of caesarean sections and increasing maternal age.62 The most frequent findings are a low-lying placenta attached on a former caesarean scar. Inside the placenta, long narrow vascular channels are visible that can be mistaken for benign placental lacunae. Normally there is no Doppler blood flow visible inside these lacunae, whereas in placenta accreta these vascular channels show high-flow velocity.63
Another very important issue in third-trimester ultrasound is the fetal birthweight prediction. There is a wide variation in the formulae for calculating estimated fetal birthweight.64 The most frequently used parameters are the abdominal circumference in combination with femur length and head circumference or biparietal diameter of the head.65–67 The likelihood ratio for prediction of newborns above 4000 g in some studies was between 2.668 and 32.66 These differences show quite plainly that the head and abdominal circumferences must be extended with the femur length, but even the best formulae are of limited value for the accurate prediction of babies beyond 4000 g.
Umbilical artery Doppler studies have been used for many years to assess fetal well-being.69 Umbilical blood velocity waveforms indirectly assess placental vascular resistance and therefore abnormal waveforms may serve as an index of placental insufficiency. In many countries umbilical Doppler is used to a large extent in both low- and high-risk pregnancies so that it seems to be indispensable in modern prenatal diagnosis and obstetric surveillance. Nonetheless, it has been shown in meta-analyses that it is only a moderately useful test in high-risk pregnancies to predict mortality and fetal compromise.70,71
Prenatal diagnosis and especially sonographic prenatal screening constitutes an increasingly stronger part of modern obstetrics. In some fields, such as in first-trimester NT Down syndrome screening, second-trimester fetal anomaly scanning, and third-trimester determination of placental localization, it has reached very high accuracy. In other areas, such as fetal weight estimation or assessment of fetal well-being, prenatal ultrasound still leaves something to be desired. In total, ultrasound is a reasonably cheap and reliable method; this and the fact that ultrasound machines are available and present in every part of the world make sonography the most important tool in modern obstetrics. The quality of ultrasound machines is still increasing. This enables us to do the most profitable tests in the first trimester of pregnancy which often lead to a completely new understanding and treatment of risk pregnancies.