I. Ultrasonography in early pregnancy During early pregnancy, ultrasonography can be used to confirm intrauterine pregnancy, except ectopic pregnancy and fetal abortion, verify the gestational week, diagnose multiple pregnancy, exclude gravidity, and understand the condition of the uterus and adnexa according to clinical needs. 1. Diagnosis of normal and abnormal pregnancies Ultrasound can be performed at the earliest transvaginal examination at 4 to 5 weeks of gestation and the corresponding human chorionic gonadotropin (HCG) > 1,500 U/L to detect the intrauterine gestational sac. In 2013, the American Society of Ultrasound Radiologists proposed criteria for the ultrasound evaluation of embryonic arrest, including: fetal bud ≥7 mm without heartbeat; mean diameter of the gestational sac (MDS) ≥25 mm without embryo; examination of A gestational sac without a yolk sac is not seen after 2 weeks; a gestational sac with a yolk sac is still not seen after 11 d. The diagnosis of embryonic abortion can be made on the basis of any of the above. 2. To determine the nature of the chorionic membrane in twin or multiple pregnancies If twin or multiple pregnancies are detected by early pregnancy ultrasonography, the nature of the chorionic membrane should be clarified, and the determination of the nature of the chorionic membrane is very important for the prediction of complications in twin pregnancies. From 7 to 10 weeks of gestation, the nature of the chorionic membrane in twin pregnancies is determined mainly on the basis of the number of gestational sacs, amniotic membrane and yolk sac. The number of gestational sacs correlates with the nature of the chorionic villus, the number of yolk sacs correlates with the nature of the amnion, and the diagnosis of monochorionic monoamniotic sac requires the identification of a single amniotic sac cavity. From 11 to 14 weeks of gestation, the nature of the chorionic membrane is mainly determined by the double fetal peak sign, with a positive predictive value of 97%. 3.Fetal nuchal translucency thickness (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. Large-scale studies have shown that for Downs syndrome, the detection rate is 70% and 64% at 11 and 13 weeks of gestation, respectively, using NT ≥ 2.5 mm as the criterion; combined with early pregnancy serologic markers, the detection rate is 87%, 85% and 82% at 11, 12 and 13 weeks of gestation, respectively, which is more accurate than midtrimester serologic screening. Ultrasound examination in middle pregnancy 1.Fetal systemic ultrasound screening from 20 to 24 weeks of pregnancy At this stage, the structure of most of the fetal organs has been fully developed, which is the best time to conduct fetal malformation examination. 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 limbs. According to the Administrative Measures of Prenatal Diagnostic Techniques of the National Health and Family Planning Commission (former Ministry of Health), the six major types of malformations for initial screening are: anencephaly, severe brain bulge, severe open spina bifida, severe chest and abdominal wall defect with visceral exostosis, single-chambered heart, and lethal chondrodysplasia. 2. Ancillary prenatal diagnosis CC screening for chromosomal abnormalities Chromosomal abnormalities in pregnancy are mostly seen in aneuploidy, such as trisomy 21, trisomy 18 and trisomy 13. For high-risk pregnant women, the diagnosis can be confirmed by ultrasound-guided chorionic villus biopsy, amniocentesis and cord blood aspiration. In addition, many fetal structural abnormalities found during midtrimester ultrasound are associated with aneuploidy, which are called ultrasound “soft indicators”. Fetal ultrasound “soft indicators” are variants of normal anatomy and are usually not clinically significant in isolation, but the higher the number of positive indicators, the more likely the fetus is to have an aneuploid chromosomal abnormality. Common midtrimester fetal ultrasound soft indicators: choroid plexus cyst, absent or underdeveloped nasal bone, intracardiac strong echogenic spot, mild renal pelvis dilatation, short long bones, fetal neck skin degree (NF), strong echogenicity of intestinal canal, iliac wing angle, single umbilical artery, etc. 3.Limitations of ultrasound screening and diagnosis of fetal malformation 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 completely displayed clearly, and the ultrasound performance of some fetal malformations changes with the increase of gestational week, thus the accuracy of ultrasound diagnosis is affected, and the detection rate of fetal malformation cannot reach 100%. 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%. In this regard, it is necessary for clinicians to fully communicate and explain with mothers to avoid unnecessary medical disputes. The accurate calculation of the gestational week is essential for the clinical management of obstetrics. Especially when the gestational week cannot be calculated according to the last menstrual period, ultrasound examination in early and middle pregnancy is a reliable means to check the gestational week. In middle and late pregnancy, the fetal weight can be estimated by ultrasound measurement of fetal diameter, and the fetal growth and development can be monitored according to the weight assessment formula or control table, with a margin of error of 16%-20%. If the estimated weight is lower than 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. Diagnosis of fetal appendage abnormalities Common fetal appendage abnormalities include single umbilical artery, umbilical cord entanglement, placenta praevia, placental implantation, placental abruption, etc. Prenatal ultrasound, especially transvaginal ultrasound, can accurately determine the position of the placenta in relation to the endocervix, so as to diagnose placenta praevia and placenta hypoplacenta. It is worth noting 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. In pregnant women with placenta previa or hypoplasia along with a history of previous uterine surgery, especially cesarean delivery, the possibility of placental implantation should be excluded. The accuracy of ultrasound observation of placenta abruptio is about 50%, therefore, when there is a high clinical suspicion of placenta abruptio, especially in acute cases, the results of ultrasound examination should not be overly relied on. The fetus can be evaluated by ultrasound through biophysical monitoring (BPP), including: fetal respiratory movement, muscle tone, fetal movement, amniotic fluid volume and fetal responsiveness (fetal heart monitoring). there is a significant linear negative correlation between BPP score and perinatal morbidity. 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 measurement of middle cerebral artery flow is usually used to evaluate the intrauterine condition of the fetus with suspected growth restriction, and peak cerebral artery flow velocity (PSV) is used to evaluate fetal anemia. 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. The normal cervical length was >30 mm throughout pregnancy, whereas pregnant women with a cervical length <25 mm had a significantly increased risk of preterm delivery. The shorter the cervix, the higher the risk of preterm birth, with the risk of preterm birth 11 times higher in pregnancies with cervical length ≤15 mm than in those with normal cervical length. The best time to predict preterm delivery is between 16 and 24 weeks of gestation.