Ultrasound in the first trimester ======================== The indications for **sonography in the first trimester** are given in the table (click the table). To check the early stages of pregnancy, you can use **transabdominal, transvaginal ultrasound** or both. The breech-seated length (CRL) is the most accurate biometric predictor of gestational age. CRL is measured in the sagittal plane and the yolk sac and organ bud should not be considered in its calculation. With **ultrasound in the first trimester**, it is possible to reliably diagnose non-embryonic pregnancy, fetal death, ectopic pregnancy and gestational trophoblastic disease. With ultrasound in the first trimester, multiple pregnancy can be detected early, and this is the best time to determine chorionicity. Also, the first trimester is a good time to examine the uterus, adnexa, and choledochal sac. Cervix length and the relationship of the placenta with the cervical opening can be examined best in the second trimester. With **transvaginal ultrasound**, it is possible to reliably observe the intrauterine pregnancy sac in the **fifth week**, and the fetus can be seen with heart activity in the sixth week. After the average diameter of the sac reaches 20 mm, the fetus must be seen in **transvaginal ultrasound**, otherwise, the pregnancy will be of the type **no embryo**. In **transvaginal ultrasound**, the movement of the fetal heart is usually seen when the length of the fetus reaches 5 mm. If no cardiac activity is detected in an embryo smaller than 7 mm, a follow-up examination within 1 week is recommended.