The basic ultrasound examination in the first trimester should include:

• location of the pregnancy (intrauterine or ectopic)
• measurement of the crown-rump length (CRL); or the gestational sac diameters if CRL is <1 cm.

NOTE: Although CRL is tightly related to age of the embryo, relationship of average sac diameter to embryonic age is quite variable. Therefore, CRL of the embryo or later fetal measurements should be used in preference to sac measurements for gestational age assessments.

• evaluation of fetal number
• location of the placenta
• evaluation of the fallopian tubes and ovaries

The yolk sac is seen within the echolucent gestational sac and is actually extraamniotic in location. As the pregnancy progresses, the embryo is clearly identified within the amniotic sac which is, in turn, within the chorionic sac, separated from the chorion by the extraembryonic coelom. The embryonic crown-rump length (CRL) can be reliably measured when it is at least 1.0 cm. Before the end of the first trimester, the fetal extremities and the fetal head can be identified.

Although not diagnostic in itself, observation of the internal os and measurement of the length of the cervical canal near the end of the first trimester may be of value in assessing competence of the cervix. Visualization of the ovaries may show small cysts related to ovulation which are transient and benign. Larger cysts should undergo serial observation to decide management.

Inherent in any examination is the evaluation of any deviations from the normal anatomy. Examples of some abnormalities which might be encountered in the first trimester can be seen on first trimester abnormalities link to the left.