Early pregnancy A gestational sac can be reliably seen on transvaginal ultrasound by 5 weeks' gestational age (approximately 3 weeks after ovulation). The embryo should be seen by the time the gestational sac measures 25 mm, about five-and-a-half weeks. The heartbeat is usually seen on transvaginal ultrasound by the time the embryo measures 5 mm, but may not be visible until the embryo reaches 19 mm, around 7 weeks' gestational age. Coincidentally, most miscarriages also happen by 7 weeks' gestation. The rate of miscarriage, especially threatened miscarriage, drops significantly after normal heartbeat is detected, and after 13 weeks. File:Ultrasound of embryo at 5 weeks.Contents in the cavity of the uterus seen at approximately 5 weeks of gestational age. File:Ultrasound of embryo at 5 weeks, colored.Artificially colored, showing gestational sac, yolk sac and embryo (measuring 3 mm as the distance between the + signs). File:Heartbeat at 5 weeks 1 day.Embryo at 5 weeks and 1 day of gestational age (at top left) with discernible heartbeat. File:Embryo at 5 weeks 5 days with heartbeat.Embryo at 5 weeks and 5 days of gestational age with discernible heartbeat. First trimester In the first trimester, a standard ultrasound examination typically includes: * Gestational sac size, location, and number * Identification of the embryo and/or yolk sac * Measurement of fetal length (known as the crown-rump length) * Fetal number, including number of amnionic sacs and chorionic sacs for multiple gestations * Embryonic/fetal cardiac activity * Assessment of embryonic/fetal anatomy appropriate for the first trimester * Evaluation of the maternal uterus, tubes, ovaries, and surrounding structures * Evaluation of the fetal nuchal fold, with consideration of fetal nuchal translucency assessment Second and third trimester In the second trimester, a standard ultrasound exam typically includes: * Fetal number, including number of amnionic sacs and chorionic sacs for multiple gestations * Fetal cardiac activity * Fetal position relative to the uterus and cervix * Location and appearance of the placenta, including site of umbilical cord insertion when possible * Amnionic fluid volume * Gestational age assessment * Fetal weight estimation * Fetal anatomical survey * Evaluation of the maternal uterus, tubes, ovaries, and surrounding structures when appropriate Dating and growth monitoring Biparietal diameter is taken as the maximal transverse diameter of in a visualization of the horizontal plane of the head. Biparietal diameter (the transverse diameter of the head) by gestational age, with the blue line representing the mean and the green area representing the 90% prediction interval. Gestational age is usually determined by the date of the woman's last menstrual period, and assuming ovulation occurred on day fourteen of the menstrual cycle. Sometimes a woman may be uncertain of the date of her last menstrual period, or there may be reason to suspect ovulation occurred significantly earlier or later than the fourteenth day of her cycle. Ultrasound scans offer an alternative method of estimating gestational age. The most accurate measurement for dating is the crown-rump length of the fetus, which can be done between 7 and 13 weeks of gestation. After 13 weeks of gestation, the fetal age may be estimated using the biparietal diameter (the transverse diameter of the head, across the two parietal bones), the head circumference, the length of the femur, the crown-heel length (head to heel), and other fetal parameters. Dating is more accurate when done earlier in the pregnancy; if a later scan gives a different estimate of gestational age, the estimated age is not normally changed but rather it is assumed the fetus is not growing at the expected rate. The abdominal circumference of the fetus may also be measured. This gives an estimate of the weight and size of the fetus and is important when doing serial ultrasounds to monitor fetal growth. Fetal sex discernment Sonogram of male fetus, with scrotum and penis in center of image The sex of the fetus may be discerned by ultrasound as early as 11 weeks' gestation. The accuracy is relatively imprecise when attempted early. After 13 weeks' gestation, a high accuracy of between 99% and 100% is possible if the fetus does not display intersex external characteristics. The following is accuracy data from two hospitals: Gestational Age King's College Hospital Medical School Taipei City Hospital & Li Shin Hospital 11 weeks 70.3% 71.9% 12 weeks 98.7% 92% 13 weeks 100% 98.3% Influencing factors The accuracy of fetal sex discernment depends on: * Gestational age * Precision of sonographic machine * Expertise of the operator * Fetal posture Ultrasonography of the cervix Fetus at 14 weeks (profile) Fetus at 14 weeks with advanced imaging filters<unk>  sonography is useful in the assessment of the cervix in women at risk for premature birth. A short cervix preterm is associated with a higher risk for premature delivery: At 24 weeks' gestation, a cervix length of less than 25 mm defines a risk group for spontaneous preterm birth. Further, the shorter the cervix, the greater the risk. Cervical measurement on ultrasound also has been helpful to use ultrasonography in patients with preterm contractions, as those whose cervical length exceeds 30 mm are unlikely to deliver within the next week. Abnormality screening In most countries, routine pregnancy sonographic scans are performed to detect developmental defects before birth. This includes checking the status of the limbs and vital organs, as well as (sometimes) specific tests for abnormalities. Some abnormalities detected by ultrasound can be addressed by medical treatment in utero or by perinatal care, though indications of other abnormalities can lead to a decision regarding abortion. Perhaps the most common such test uses a measurement of the nuchal translucency thickness ("NT-test", or "Nuchal Scan"). Although 91% of fetuses affected by Down syndrome exhibit this defect, 5% of fetuses flagged by the test do not have Down syndrome. Ultrasound may also detect fetal organ anomaly. Usually scans for this type of detection are done around 18 to 23 weeks of gestational age (called the "anatomy scan", "anomaly scan," or "level 2 ultrasound"). Some resources indicate that there are clear reasons for this and that such scans are also clearly beneficial because ultrasound enables clear clinical advantages for assessing the developing fetus in terms of morphology, bone shape, skeletal features, fetal heart function, volume evaluation, fetal lung maturity, and general fetus well being. Second-trimester ultrasound screening for aneuploidies is based on looking for soft markers and some predefined structural abnormalities. Soft markers are variations from normal anatomy, which are more common in aneuploid fetuses compared to euploid ones. These markers are often not clinically significant and do not cause adverse pregnancy outcomes.
Obstetric ultrasonography
Fertility testing
Vaginal ultrasonography
Postcoital test
Gynecologic ultrasonography
Hysterosalpingography
Karyotype
Fetal echocardiography