A fetal pole is a collection of fetal cells that can be detected via vaginal ultrasound around the sixth week of pregnancy. Separate from the yolk sac, it is considered the somite stage of the fetus.
The fetal pole grows at a steady rate of about 1 millimeter per day, starting in the week of gestation. For this reason, the fetal pole is used to accurately predict the age of the fetus through ultrasound, by adding the length of the fetus in millimeters to six weeks. For example, a fetal pole measuring 4 millimeters in length has an approximate age of six weeks and four days.
The fetal pole is considered to first visible sign of a fetus in development. Typically, the fetus's head is at one end of the pole and what appears to be its "tail" at the other end, lending a curved appearance to the pole.
The fetal pole is used to determine the viability of a pregnancy in many cases. The fetal heartbeat can usually be detected once the pole reaches 5 millimeters in length. If a vaginal ultrasound is performed at this stage and no heartbeat is detected, it can mean a possibility of miscarriage. Another ultrasound should be performed within a week to rule out any complications. Pregnancy dating is inexact; therefore, determining a clear outcome is highly unlikely until the pregnancy is further along.