Ultrasound screening for pregnancy abnormalities in the third trimester is a specialized investigation which should be considered a routine/standardized examination in antenatal care, if resources are available and accessible.
Limited obstetric ultrasound is usually performed as a response to a particular clinical situation and in cases of pregnancies which did not benefit from third-trimester screening ultrasound for pregnancy abnormalities. This will be scheduled as soon as possible(1).
According to the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) and The World Federation of Ultrasound in Medicine and Biology (WFUB), no case of fetal anomaly due to excessive use of ultrasonography has been reported in more than 40 years of its use(2,3). However, the use of ultrasound without medical benefit should be avoided; ultrasonography should be conducted only by medically trained examiners and it is recommended to reduce fetal exposure as much as possible for obtaining the required medical information (according to the principle of ALARA = As Low As Reasonably Achievable)(2,3).
The purpose of screening methods during pregnancy is to lower mortality and perinatal morbidity. Ultrasound examination is an investigation that is acceptable to the public, has an affordable cost and has the potential of depicting abnormal or atypical aspects before the occurrence of clinical manifestations.
By observing the pregnancy, the ultrasound examination allows the establishment of prevention or the therapeutic treatment in relation to prematurity, postterm pregnancy, fetal growth abnormalities, abnormal adhesion of the placenta with insertion on the caesarean section scar etc.
The ultrasound examination allows the reliable recognition of multiple pregnancies, amnionicity and chorionicity, but their examination is not reliable in the third trimester. This information is essential for the management of potential risks and complications.
Ultrasound can identify curable pathologies associated with pregnancy. Therefore, these findings can sometimes lead to changes in pregnancy management by shifting the focus towards specialized antenatal care centers or to advising patients to give birth in centers with the most adapted technical and logistical possibilities.
Fetal pathologies incompatible with life or incurable may also be detected, according to the medical information known at the time and in the context of detection, with consequences on the maternal-fetal care/therapeutic decisions.
It is wrong to interpret the normal results of the ultrasound in pregnancy as a “normality certificate of the newborn or of the child”, because not all abnormalities can be detected through ultrasound examination. No screening process will detect all anomalies which are theoretically identifiable; a “normal” ultrasound examination (i.e., no abnormalities detected) does not guarantee the absence of a serious pathology. There are fetal abnormalities that cannot be detected at the antenatal ultrasound examination or abnormalities with late/progressive ultrasound appearance/semiology which are not visible at the time of the examination. This specification must be found on the ultrasound report.
The ultrasound cannot exclude chromosomal abnormalities and genetic disorders.
The objectives of this ultrasound examination are to:
confirm live fetus;
determine the number of live fetuses (and in the case of multiple pregnancies, to determine the number of amnionicity and try to specify chorionicity);
assess fetal presentation;
assess fetal anatomy in relation to gestational age;
determine fetal size and estimate fetal growth by age (corrected according to the patient’s medical records);
assess maternal anatomy and pregnancy annexes for an obvious pathology with adverse consequences.
Why a third-trimester ultrasound as a screening method?
The detection of fetal growth restriction, of previously undetected/evolving fetal abnormalities in the second trimester, the pathology and location of the placenta are situations that may have an impact on the medical conduit during the last trimester of pregnancy and on delivery decision.
Observational studies indicate that small-for-gestational-age (SGA) newborns and especially newborns with fetal growth restriction (FGR)(4,5) are the ones most exposed to developing complications. About 400,000 pregnancies a year can be complicated with FGR in Europe, with a cohort study indicating that 5.7% of fetuses have late FGR(6). On the other hand, fetuses which are assessed as large for gestational age (LGA) might be associated with pathological pregnancies and may generate dystocic deliveries. Introducing a protocol for the assessment of fetal weight, combined with sonographers and their audit training, demonstrated a significant decrease in the number of stillbirths(7,8). Fetal biometry, BPD (biparietal diameter), CC (cranial circumference), AC (abdominal circumference) and FL (femur length) have higher value if performed in two separate gestational age groups, namely at 28-30 and 34-36 gestational weeks(9), for the detection of fetal growth restriction.
Pregnancy screening methods, which involve a single third-trimester ultrasound (at 32 weeks), have less than 60% sensitivity and less than 30% specificity for SGA(10), while at a gestational age of 36 weeks the FGR detection rate is superior to single 32-week ultrasound (sensitivity 38.8% versus 22.5%; p=0.006)(11). The most sensitive method of fetal growth anomalies detection is the dynamic of fetal abdominal circumference growth curve, below the 10th percentile – P10(12). Generally, the longitudinal assessment of fetal growth, with reference to the initial dating ultrasound, is better than a single ultrasound assessment of fetal size(13,14) and AC is the most important part of the fetal weight estimation (EFW)(15). The reassessment of AC estimation or EFW is required every three weeks when there is a suspicion of SGA(16). The detection of small fetuses while dynamically assessing fetal growth requires further investigations(4), and particularly Doppler use(17). The use of maternal-fetal Doppler assessments may decrease perinatal mortality(17,18) and differentiate between SGA and FGR(5,17,19). So far, there is no evidence available for the low-risk pregnancy populations, suggesting that routine Doppler use on the umbilical artery or Doppler combination on uterine arteries would pose a benefit for the mother or newborn(18). On the other hand, a high resistance index or a high pulsatility index on the umbilical artery indicates an increased risk of preeclampsia or FGR, requiring further investigations(17,20,21).
Abnormalities of placental insertion and position
If these two characteristics of the placenta were not assessed as normal during the second-trimester ultrasound examination, the placenta location and its ratio with the internal cervical orifice should be described. Examples of abnormal placenta include the presence of hemorrhage or hematoma. The pregnant women with caesarean section scar, lower and anterior placental insertion or with placenta praevia present an increased risk of developing abnormal placental adhesion. In these situations, the signs of accretization should be looked for in the placenta, and the presence of multiple, irregular placental gaps, accompanied by arterial or mixed blood flows, has the greatest sensitivity. The abnormal appearance of the interface between the uterine and bladder wall is a very specific sign for adhesion pathology, but rarely seen(22). The ultrasound semiology associated with the accretization of the placenta on the caesarean section scar includes: protrusion of placental mass in the posterior bladder wall, placental gaps in the caesarean section scar area, disappearance of the sonolucent area between the placenta and the uterine wall, myometrial thinning <1 mm, interruption of the bladder hyperechogenic outline, abnormal presence of placental tissue or clot inside the urinary bladder, Doppler pulsatile signs on the maternal site of the placenta (especially adjacent to or inside the placental gaps), and maternal blood flow arising from the arched arteries and penetrating into the placenta.
The accuracy of the third-trimester diagnosis is encumbered by suboptimal images, given the impossibility of maintaining a full urinary bladder. Transvaginal ultrasound, possibly combined with color Doppler, may be helpful(23).
Approximately 14% of structural abnormalities were additionally found during the third-trimester ultrasound examination after first- and second-trimester screening(24). The evaluation of fetal anatomy in the third trimester is performed in the same way as during the second-trimester pregnancy, with limitations given by fetal position, especially for the face, sacral bone and extremities(24). An ultrasound screening guideline for pregnancy abnormalities of the fetal anatomy in the second trimester is in effect in Romania(25).
Several recommendations and guidelines for third-trimester ultrasound examination have been published by various professional societies(1,4,8,16,26-29).
Ultrasound examination for the third-trimester screening should be performed between 30 weeks and 35 weeks + 6 days of gestation.
The ultrasound third-trimester screening for pregnancy abnormalities should include:
a) The confirmation of the live fetus. Confirmation of the live fetus is done by highlighting the fetal cardiac movements (FCM) in real time and measuring their frequency in M or Doppler pulsed mode (used as shortly as possible). In a multiple pregnancy, these should be evaluated in each fetus.
b) The determination of the number of live fetuses (in the multiple pregnancies, amnionicity and chorionicity should be determined, with reference for second opinion if it fails).
c) Fetal presentation. In a multiple pregnancy, the presentation should be assessed in each fetus.
d) The assessment of the pregnancy anatomy – all items considered part of a routine in this guideline and discussed below should be evaluated. Due to limitations of the third trimester given by fetal position and size, maternal conditions (e.g., obesity, inferior vena cava syndrome etc.), the evaluation is reduced to a minimal level: the symmetry of the middle line of the skull, the cerebral ventricles, the 4-chamber heart and the three-vessel-trachea section (optionally with Doppler color), the diaphragm, the anechoic stomach, kidneys, urinary bladder. If some elements cannot be properly assessed, the patient must be made aware of this problem, offering the option of their subsequent reevaluation or of a second opinion referral being often appropriate. In a multiple pregnancy, the anatomy should be evaluated in each fetus.
Moreover, other elements may be assessed in accordance with the legal competences, expertise and experience of the examiner, the equipment used and the medical motivation or indication.
For the evaluation of fetal development, BPD (biparietal diameter), HC (head circumference), AC (abdominal circumference) and FL (femoral diaphysis length of the proximal femur) should be measured using the ultrasound probe.
e) Fetal annexes – for the most objective assessment, the placental thickness and the largest bag of amniotic fluid (AF) or the amniotic fluid index (AFI) may be optionally measured.
Placenta (location and detailed description of abnormal findings, the presence of structural abnormalities, the signaling and description of the relationship between uterine scars and signs of abnormal adhesion, where appropriate).
Amniotic fluid (appearance/echogenicity, quantity).
f) The cervix should preferably be examined transvaginally for patients with a history of premature birth or with symptoms during the current pregnancy. Measure the length of the cervical canal in millimeters.
g) Obvious abnormalities of maternal internal genitalia. In advanced pregnancy, the detailed and reliable evaluation of the maternal internal genitalia is not possible. The presence of voluminous tumors of the uterine body or cervix should be noticed.
The examination report should include the classification of the measured parameters within normality standards (appropriate diagrams may be used) and the estimate of fetal weight (EFW – it is recommended to use percentiles in accordance to INTERGROWTH-21st(30)) through the Hadlock formula, using cranial, abdominal and femoral measurements. The examination report will contain a summary of the findings of the examiner. It is recommended to describe all morphological elements in the report by using the following collocations: normal appearance/abnormal appearance/not properly evaluated. In the examination report, the collocation “normal aspect” means that the specified structure or organ has been visualized and interpreted as normal by the examiner.
However, as with all screening examinations, there may be interpretations with inaccurate results.
The imaging documentation of all specified elements for all examinations can be beneficial in the sense of reducing uncertainty and litigation.
It is not recommended to record examinations as a movie or to perform 3D/4D examinations outside scientific indications to document normality or detect fetal disorders. Still, the influence of these techniques on the psychological factor of parental connection is recognized.
The routine items which for a number of reasons have not been viewed during the examination should be consigned. The reexamination in order to clarify their normality or anomalies is considered to be beneficial and is encouraged by this guideline.
In the examination report, the examination difficulties (unfavorable fetal position, anterior location of the placenta, particular maternal anatomy such as adipose tissue, scar etc.) will also be depicted.
The anomaly findings or suspicions will be specified and the possibility of overspecialized examination or the referral for second opinion or to other medical specialists should be offered. The examiner can make remarks, specify/suggest a diagnosis and make recommendations, within the limits of his speciality and competence, within the synthesis of the examination report.