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.png|Contents in the cavity of the
uterus seen at approximately 5 weeks of
gestational age File:Ultrasound of embryo at 5 weeks, colored.png|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.gif|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.gif|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 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 14 weeks of gestation. After 14 weeks of gestation, the fetal age may be estimated using the head circumference with or without the length of the
femur.
Influencing factors The accuracy of fetal sex discernment depends on: 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. ==Safety issues==