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.Also used is Doppler sonography which detects the heartbeat of the fetus.Doppler sonography can be used to evaluate the pulsations in the fetal heart and bloods vessels for signs of abnormalities.One variant, a transvaginal sonography, is done with a probe placed in the woman's vagina.Transvaginal scans usually provide clearer pictures during early pregnancy and in obese women.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.Not useful for dating, the abdominal circumference of the fetus may also be measured.
Soft markers are variations from normal anatomy, which are more common in aneuploid fetuses compared to euploid ones.
A short cervix preterm is undesirable: At 24 weeks gestation a cervix length of less than 25 mm defines a risk group for preterm birth, further, the shorter the cervix the greater the risk.
In some 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.