One of the frequently seen abnormal first trimester ultrasound findings is a cystic hygroma. Cystic hygroma is most often noted as a separation of the skin at the back of the baby's neck from the underlying muscle and tissue with fluid accumulation in the resulting space forming cysts. Approximately 50% of fetuses with an identifiable cystic hygroma will have an underlying chromosome abnormality such as Down syndrome or Turner syndrome. A cystic hygroma can also be identified in the second trimester and are also highly suggestive of an underlying chromosome abnormality at that time. Due to this increased risk of a chromosome abnormality for the fetus, an amniocentesis or CVS may be beneficial.

Ectopic pregnancies may be suspected by noting no intrauterine gestational sac in the face of a sufficiently elevated serum beta-HCG, or may be diagnosed on ultrasound with the visualization of a gestational sac in an extrauterine location, often in the fallopian tubes.

Ultrasound examination in the first trimester of a twin pregnancy represents a limited opportunity to identify identical twinning, along with the potential clinical implications of this type of pregnancy. Twin gestations in the first trimester may be classified as to the number of chorions and amnions involved in the pregnancy and thus, some idea of mono- or dizygosity emerge (identical or fraternal twins). If a twin pregnancy is monochorionic (has only one chorionic membrane surrounding both embryos), then it must have derived from only one fertilized egg (identical twins). If it is dichorionic (two chorionic membranes), then both embryos could have derived from one fertilized egg or the embryos could be derived from separate eggs (dizygous or fraternal twins).

When an embryo dies in the first trimester (as frequently happens naturally), the embryo and even the entire sac may grow smaller and disappear in ultrasound. When one of twin embryos dies, the phenomenon is often called a vanishing twin.

In any first trimester examination, counting the number of sacs and embryos is critical, as counting babies in utero when they are larger is quite difficult. We carefully sweep from one side of the uterus to the other, visualizing each sac in a continuous sequence.