Why is it important to scan the adnexa and CDS during a first trimester exam?

Scanning the adnexa and the cul-de-sac (CDS) during a first trimester exam is crucial primarily to rule out ectopic pregnancy or heterotopic pregnancy. Ectopic pregnancies occur when a fertilized egg implants outside the uterus, most commonly in a fallopian tube. Since this type of pregnancy poses significant health risks to the mother, identifying it early through ultrasound is vital.

In the first trimester, the presence of gestational tissue should be confined to the uterus. By examining the adnexa and CDS, sonographers can look for fluid, free blood, or abnormal masses that may indicate an ectopic or heterotopic pregnancy (where there is one intrauterine and one extrauterine pregnancy). The detection of any irregularities in these areas helps guide appropriate management and intervention, potentially preventing complications.

While assessing fetal size and growth, evaluating the cervix and uterus, and checking for placental abnormalities are also important aspects of obstetric ultrasound, they do not address the immediate and potentially life-threatening conditions associated with an ectopic or heterotopic pregnancy in the first trimester. Thus, the primary focus during this early stage is to confirm that the pregnancy is located within the uterus to ensure maternal safety.