Ultrasound technology has been used in obstetrics for more than 40 years and has become an essential part of pregnancy care.
Ultrasound examination during early pregnancy can be used to confirm intrauterine pregnancy, exclude ectopic pregnancy and fetal abortion, check the gestational week, diagnose multiple pregnancy, exclude gravidity, and understand the condition of the uterus and adnexa according to clinical needs.
(a) Diagnosis of normal and abnormal pregnancy ultrasound can be performed as early as 4 to 5 weeks of gestation by transvaginal examination, with the corresponding human chorionic gonadotropin (HCG) >1500 U/L to detect the intrauterine gestational sac. If there is no yolk sac or germ in the gestational sac, care should be taken to differentiate it from an echogenic dark area in the uterus, as an echogenic dark area can be detected in the uterine cavity in ectopic pregnancy, while the typical intrauterine gestational sac is the double ring sign.In 2013, the American Society of Ultrasound Radiologists proposed criteria for ultrasound evaluation of embryonic arrest, including: gestational bud ≥7 mm without heartbeat; mean diameter of gestational sac (MDS) ≥25 mm without embryo; examination of A gestational sac without a yolk sac was not seen to have a heartbeat after 2 weeks; a gestational sac with a yolk sac was not seen to have a heartbeat after 11 d of examination. The diagnosis of embryonic abortion can be made on the basis of any of the above. For ectopic pregnancy it should be noted that the detection rate of ectopic pregnancy is 40.9% to 76.0% with transabdominal ultrasound and 75.6% to 95.8% with transvaginal ultrasound. The diagnosis of ectopic pregnancy requires a combination of medical history, blood HCG levels and dynamic monitoring by ultrasound.
(b) Determining the nature of the chorionic membrane in twin or multiple pregnancies If twin or multiple pregnancies are detected on early ultrasound, the nature of the chorionic membrane should be clarified, and the determination of the nature of the chorionic membrane is important for the prediction of complications in twin pregnancies.
(c) Fetal nuchal translucencythickness (NT) screening at 11-13+6 weeks of gestation NT is the normal fluid-filled gap between the fetal nuchal area and the surface skin, and thickening is directly associated with fetal aneuploidy chromosomal abnormalities such as trisomy 21, trisomy 18, trisomy 13 and major structural malformations such as cardiac malformations. NT measurements are performed between 11 and 13+6 weeks of gestation. Currently, NT values are used in combination with serologic indicators to calculate the risk of Down’s syndrome, and if the risk is high, invasive prenatal diagnosis is recommended. If NT ≥ 3.0 mm is the standard thickening, then invasive prenatal diagnosis is directly recommended. In addition to chromosomal abnormalities, NT thickening is also significantly associated with the incidence of other fetal anomalies, such as fetal precocious heart disease, skeletal abnormalities, and genetic syndromes, etc. The fetal anomaly rate is 8% for NT 3.0-3.4 mm, 17% for 3.5-4.4 mm, 29% for 4.5-5.4 mm, 64% for 5.5-6.4 mm, and 64% for ≥3.0 mm. The fetal abnormality rate is 64% for those with 5.5-6.4 mm, and increases to 80% for those with ≥6.5 mm.
Causes of NT thickening may include heart failure secondary to structural malformations, extracellular matrix abnormalities, abnormal lymphatic system development, anemia, and congenital infections.
NT screening may also detect serious fetal anomalies such as anencephaly, anencephaly, hydrocystic lymphangiectasia, severe congenital heart disease, skeletal anomalies, and umbilical bulge. The NT screening may also detect serious fetal anomalies such as anencephaly, anencephaly, hydrocystic lymphadenopathy, severe congenital heart disease, skeletal abnormalities, umbilical bulge and abdominal wall cleft, and giant bladder.
Ultrasound examination in middle pregnancy
(a) Systematic ultrasound screening of fetus from 20 to 24 weeks of gestation At this stage, most of the fetal organs are fully developed and it is the best time to check for fetal malformations. Systematic examination of the fetus from head to toe can detect malformations of the head and neck, heart and lungs, digestive system, genitourinary system and extremities. The six major types of malformations initially screened for are: anencephaly, severe brain bulge, severe open spina bifida, severe chest and abdominal wall defect with visceral exostosis, single-chambered heart, and lethal chondrodysplasia, as stipulated in the Measures for the Administration of Prenatal Diagnostic Techniques of the National Health and Family Planning Commission (formerly the Ministry of Health).
Midtrimester systemic ultrasound screening includes.
(1) number of fetuses.
(2) Fetal orientation.
(3) fetal heart rate.
(4) Fetal biology measurements.
(1) biparietal diameter.
(2) head circumference.
(3) transverse cerebellar diameter.
(iv) femur length.
(⑤) abdominal circumference.
(5) Fetal anatomical structures.
(①fetal skull: cranial strong echogenic ring, cerebral hemispheres, midline, lateral ventricles, thalamus, cerebellar hemispheres, cerebellar earth, posterior cranial fossa pool.
(ii) Fetal face: continuity of the upper lip.
③Fetal neck: presence of masses and skin edema.
④fetal chest: both lungs, heart position.
⑤ fetal heart: four-chamber heart section, left ventricular outflow tract section, right ventricular outflow tract section, fetal echocardiography is recommended for suspected fetal cardiac macrovascular malformation.
(6) Fetal abdomen: abdominal wall, liver, stomach, both kidneys, bladder, and the entrance to the abdominal wall of the umbilical cord.
(7) Fetal spine: sagittal section of the spine to observe the continuity of the spine, coronal section and transverse section scan if necessary.
(8) Fetal limb long bones.
(6) Fetal appendages.
①Placenta and umbilical cord: placenta position, thickness, placenta maturity, number of umbilical cord vessels.
(ii) Amniotic fluid volume: the maximum depth of amniotic fluid or amniotic fluid index to assess the volume of amniotic fluid.
(7) Maternal uterus: endocervical opening, uterine fibroids and other conditions.
(2) Ancillary prenatal diagnosis CC screening for chromosomal abnormalities common midtrimester fetal ultrasound soft indicators of choroid plexus cysts, absent or underdeveloped nasal bones, intracardiac strong echogenic light spots, mild renal pelvis dilatation, short long bones, fetal neck skin thickness (NF), strong echogenicity of intestinal canal, iliac wing angle, single umbilical artery, etc.
A Meta-analysis showed that the likelihood ratios for single soft indicators to predict the risk of trisomy 21 occurrence were 6.58 for absent or hypoplastic nasal bone, 3.94 for right subclavian artery variant, 3.81 for widened lateral ventricle, 3.79 for NF ≥6 mm, 1.65 for strong echogenicity of the intestinal canal, 1.08 for mild renal pelvis dilatation (≥4 mm), 0.95 for strong echogenicity in the heart, and short femur was 0.78, short humerus was 0.67, and choroid plexus cysts did not increase the risk of trisomy 21.
(C) Limitations of ultrasound screening and diagnosis of fetal malformations Ultrasound screening is limited by the fetal gestational week, body position, amniotic fluid volume, maternal abdominal wall thickness, ultrasound equipment and examiner’s experience, etc. Some structures cannot be fully displayed clearly, and the ultrasound performance of some fetal malformations changes with increasing gestational week, thus the accuracy of ultrasound diagnosis is affected, and the detection rate of fetal malformations cannot reach The detection rate of fetal anomalies cannot reach 100%.
The sensitivity of ultrasound screening for fetal anomalies ranges from 13.3% to 82.4%, with an average of 40.4%.
Currently, the prenatal ultrasound detection rates of some fetal anomalies reported in domestic and international literature are as follows.
61%-95% for open spina bifida; 26.6%-92.5% for fetal cleft lip and palate; 0-1.4% for simple cleft palate; about 60.0% for diaphragmatic hernia; 0-5.0% for atrial septal defect; 0-66.0% for ventricular septal defect; 14.0%-65.0% for tetralogy of Fallot; 28.0%-95.0% for hypoplastic left heart syndrome; 9.2%-57.1% for gastrointestinal anomalies The percentage of fetal limb malformation is 22.9%~87.2%.
(3) Judgment and understanding of fetal growth and development
(a) Checking the gestational week is crucial to the clinical management of obstetrics. Especially when the gestational week cannot be projected from the last menstrual period, ultrasound examination in early and middle pregnancy is a reliable means to check the gestational week. Indicators used to assess the gestational week during early pregnancy include MSD and cephalopelvic length (CRL). Before 6 weeks of gestation, gestational days = MSD + 30; from 6 to 10 weeks of gestation, gestational week is most accurately predicted based on CRL, which can be accurate within 3-5 d. When CRL > 60 mm, fetal biometry values of biparietal diameter, head circumference, abdominal circumference, and femoral length can be used to evaluate fetal size and verify gestational week. Among them, head circumference is the most accurate, which can be accurate to 3.4d at 14-22 weeks.
(b) To understand fetal growth and development in middle and late pregnancy, the fetal weight can be estimated by ultrasound measurement of each fetal diameter, according to the weight assessment formula or control table, so as to monitor fetal growth and development with a margin of error of 16% to 20%. If the estimated weight is below the 10th percentile of the corresponding gestational week, intrauterine growth restriction should be considered; if the estimated weight exceeds 4000 or 4500 g, the possibility of a giant baby should be considered. The more sensitive indicators for estimating fetal weight are fetal abdominal circumference and femoral length. Although fetal head circumference and biparietal diameter have some correlation with fetal weight, the correlation is lower compared to abdominal circumference, and the increase in fetal weight in late pregnancy is mainly due to the accumulation of liver glycogen and fat. In addition, the accurate measurement of fetal head circumference in late pregnancy is influenced by fetal orientation [11]. For intrauterine growth restriction, ultrasound should be repeated every 2 to 4 weeks for monitoring and to know the amniotic fluid volume and monitor the umbilical artery blood flow. At this stage, attention should also be paid to the examination of fetal anatomy, except for some anomalies that were not detected or were not obvious in early and mid-trimester.
The diagnosis of fetal appendage abnormalities is common: single umbilical artery, umbilical cord entanglement, placenta praevia, placental implantation, placental abruption.
Prenatal ultrasound, especially transvaginal ultrasound, can accurately determine the position of the placenta in relation to the endocervix, and thus diagnose placenta praevia and placenta hypoplacenta. It is important to note that placenta previa and hypoplastic state are common in mid-trimester and can be definitively diagnosed if they are still present after 28 weeks of gestation. The possibility of placenta implantation should be excluded in pregnant women with placenta previa or hypoplasia along with a history of previous uterine surgery, especially cesarean delivery. The most sensitive ultrasound manifestation for the diagnosis of placenta implantation is an irregular cavity in the placenta with arterial or mixed arteriovenous flow; abnormal echogenicity at the uterine bladder junction is a specific manifestation of placenta implantation. In addition, there is also the disappearance of the hypoechoic zone between the placenta previa portion and the myometrium. The accuracy of ultrasound observation of placental abruption is about 50%, so when there is a high clinical suspicion of placental abruption, especially in acute cases, one should not rely excessively on the ultrasound findings.
V. Aids in determining intrauterine fetal well-being
(There is a significant linear negative correlation between BPP score and perinatal morbidity.
(b) Ultrasound Doppler flow monitoring of maternal uterine artery, fetal umbilical artery, middle cerebral artery, and venous ducts can help determine the intrauterine condition of the fetus. The presence of cut marks or elevated resistance indices in the uterine artery Doppler waveform may indicate pregnancy complications such as fetal growth restriction and preeclampsia. The umbilical artery Doppler waveform reflects the status of placental blood supply. Increasing end-diastolic flow, absence or reversal of end-diastolic flow in the umbilical artery with increasing gestational weeks indicates intrauterine fetal hypoxia. Doppler measurements of middle cerebral artery flow are often used to evaluate intrauterine conditions in fetuses with suspected growth restriction. Peak cerebral artery flow velocity (PSV) is used to evaluate fetal anemia, and values above 1.5 times the median indicate fetal anemia. Early pregnancy venous catheter systolic reverse flow is associated with fetal aneuploidy and cardiac malformations, and abnormal venous catheter waveforms in mid and late pregnancy are often indicative of severe intrauterine distress. Ultrasound Doppler flow monitoring is mainly used for monitoring fetuses of high-risk pregnant women with pregnancy comorbidities or complications.
VI. Prediction and diagnosis of preterm labor Transvaginal measurement of cervical length has a very high negative predictive value and specificity for the prediction of preterm labor. Normal cervical length throughout pregnancy was >30 mm, while pregnant women with cervical length <25 mm had a significantly increased risk of preterm delivery. The shorter the cervix, the higher the risk of preterm delivery, with the risk of preterm delivery at cervical length ≤15 mm being 11 times higher than in pregnant women with normal cervical length. The best time to predict preterm labor is between 16 and 24 weeks of gestation.