In the late stages of pregnancy, from 37 weeks onwards, unstable lie is the term given to a baby that continues to change its position inside the womb.
The baby's position is not classed as unstable until this point as, prior to this, the baby will move around the womb as a normal part of its development.
However, towards the end of pregnancy, most babies will move into a head-first position, which is known as the head engaging. This position will enable the baby to pass through the birth canal when labour begins. A baby in an unstable lie will continue to move vertically and horizontally.
A healthy uterus is able to accommodate an unstable lie as, even in the late stages of pregnancy, it can still stretch and flex to suit the position of the baby. So, from this perspective, it is not a cause for concern.
However, the mother's uterus should always be checked for a condition known as placenta praevia. This is where the placenta is lying too low down in the uterus. The low lying placenta therefore prevents the baby from moving into the head-first position, thus causing the unstable lie. Other issues such as fibroids and ovarian cysts can also cause an unstable lie, and a woman will also be checked for this.
If all seems well with the placenta and uterus, the woman's labour may be induced if the baby flips into a head-down position. If this is not possible, a caesarean section may be recommended instead.