Anembryonic gestation
Encyclopedia
An anembryonic gestation (aka blighted ovum) is a pregnancy
in which the very early pregnancy appears normal on an ultrasound scan
, but as the pregnancy progresses a visible embryo
never develops. In a normal pregnancy, an embryo would be visible on an ultrasound by six weeks after the woman's last menstrual period.
An anembryonic gestation is characterized by a normal-appearing gestational sac
, but the absence of an embryo. It likely occurs as a result of early embryonic death with continued development of the trophoblast
. When small, the sac cannot be distinguished from the early normal pregnancy, as there may be a yolk sac
, though a fetal pole
is not seen. For diagnosis, the sac must be of sufficient size that the absence of normal embryonic elements is established. The criteria depends on the type of ultrasound exam performed. A pregnancy is anembryonic if a transvaginal ultrasound reveals a sac with a mean gestational sac diameter (MGD) greater than 13 mm and no yolk sac, or an MGD >18 mm with no embryo. If a transvaginal exam is not performed, the criteria for a transabdominal scan is a MGD of 25 mm or more without an embryo or an MGD of 20 mm or more without a yolk sac.
Rather than do a transvaginal exam at the time of the initial visit, many centers prefer to do only a transabdominal study and offer mothers a follow-up ultrasound 10 days later to see if a normal pregnancy subsequently develops.
Anembryonic gestation is one of the causes of miscarriage
of a pregnancy
. Several management options exist for anembryonic pregnancies which have not miscarried on their own.
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
in which the very early pregnancy appears normal on an ultrasound scan
Obstetric ultrasonography
Obstetric sonography is the application of medical ultrasonography to obstetrics, in which sonography is used to visualize the embryo or foetus in its mother's uterus...
, but as the pregnancy progresses a visible embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
never develops. In a normal pregnancy, an embryo would be visible on an ultrasound by six weeks after the woman's last menstrual period.
An anembryonic gestation is characterized by a normal-appearing gestational sac
Gestational sac
The gestational sac is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy exists, until the embryo is identified....
, but the absence of an embryo. It likely occurs as a result of early embryonic death with continued development of the trophoblast
Trophoblast
Trophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg...
. When small, the sac cannot be distinguished from the early normal pregnancy, as there may be a yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
, though a fetal pole
Fetal pole
The fetal pole is a thickening on the margin of the yolk sac of a fetus during pregnancy.It is usually identified at 6.5 weeks with abdominal ultrasound imaging, and 6 weeks with vaginal ultrasound imaging. However it is quite normal for the fetal pole to not be visible until about 9 weeks...
is not seen. For diagnosis, the sac must be of sufficient size that the absence of normal embryonic elements is established. The criteria depends on the type of ultrasound exam performed. A pregnancy is anembryonic if a transvaginal ultrasound reveals a sac with a mean gestational sac diameter (MGD) greater than 13 mm and no yolk sac, or an MGD >18 mm with no embryo. If a transvaginal exam is not performed, the criteria for a transabdominal scan is a MGD of 25 mm or more without an embryo or an MGD of 20 mm or more without a yolk sac.
Rather than do a transvaginal exam at the time of the initial visit, many centers prefer to do only a transabdominal study and offer mothers a follow-up ultrasound 10 days later to see if a normal pregnancy subsequently develops.
Anembryonic gestation is one of the causes of miscarriage
Miscarriage
Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving independently, generally defined in humans at prior to 20 weeks of gestation...
of a pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
. Several management options exist for anembryonic pregnancies which have not miscarried on their own.