Abnormal first trimester findings
Cystic hygroma seen in the first trimester is most often noted as a separation of the nuchal skin from the underlying structures, leaving a posterior nuchal echolucency. Occasionally, cyst formation in the subcutaneous area is noted. Although cystic hygroma in the second trimester is highly suggestive of an underlying chromosome abnormality (monosomy X or one of the autosomal trisomies being found most frequently), only about half of the embryos found to have cystic hygroma in the first trimester are discovered to have gross chromosomal defects on chorionic villus sampling or amniocentesis. 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. A cornual pregnancy is noted here with the sac containing an embryo located adjacent to the endometrial cavity. No intervening myometrium is found in some areas where the placenta is seen contiguous with the uterine serosa.
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. If a twin pregnancy is monochorionic (has only one chorionic membrane surrounding both embryos), then it must have derived from only one fertilized egg. If it is dichorionic (two chorionic membranes), then both embryos could have derived from one fertilized egg (with early division into monozygous twins) or the embryos could be derived from separate eggs (dizygous twins).
When embryos die in the first trimester (as frequently happens naturally), the embryo and even the entire sac may grow smaller and disappear on 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. Remember that, when you encounter triplets, suspect quadruplets.
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. Remember that, when you encounter triplets, suspect quadruplets.
