In about 5% of routine 18 - 20 week pregnancy ultrasounds, the placenta is seen to be lying low. Meaning the placenta is situated lower in the uterus, closer to the woman's cervix. A low-lying placenta at this stage of pregnancy is regarded as normal because the lower segment of the uterus has not fully formed yet. In about 90% of cases, the previously low-lying placenta moves up into the upper segment as the uterus grows, meaning that by about the 34th week it is no longer low-lying. On rare occasions (0.5% of pregnancies), the placenta remains in the lower segment of the uterus, sometimes growing to the extent that it covers the woman's cervix. This is known as Placenta Previa, meaning 'placenta first’.
Lower uterine segment
NurFakhriyyah, LokemanHazli, Alia NadhirahAssociate Professor Dr Hanifullah Khan 1 03/23/12
* This presentation aims to * Explain what the Lower Uterine Segment (LUS) is * An outline of it’s anatomical and histological features * The implications of the LUS in clinicalpractice 2 03/23/12
* Uterine enlargement in pregnancy * by hypertrophy, hyperplasia and stretching of the muscle fibres. * from 50-60g to almost 1000g at term* At 7 weeks - the size of an egg;* At 10 weeks it is the size of an orange and fills the entire pelvis.* At 12 weeks it rises out the pelvis and palpable abdominally just above the symphysis pubis 3 03/23/12
* The lower uterine segment (LUS) is the part of the uterus and the upper cervix which lies between the attachment of the peritoneum of the uterovesical pouch superiorly & the internal cervical os inferiorly.* The isthmus of the uterus, the lower extremity of which joins with the cervical canal and during pregnancy expands to become the lower part of the uterine cavity 4 03/23/12
* With increasing gestation, there is increased stretching of the uterine muscle fibres.* This results in the formation of the lower segment in the 3rd trimester (28 – 40 weeks). 5 03/23/12
* This part of the uterus is thinner contain less muscle fibres and blood vessels.* This is the site for caesarean section incision.* The placenta is attached to the more vascular and thicker upper segment. 7 03/23/12
* In such cases, the placenta is attached partially or completely to the LUS* Placental a t t a c h m e n t i s d i s r u p t e d as this area gradually thins in preparation for the onset of labor. * bleeding occurs at the implantation site as the uterus is unable to contract adequately and stop the flow of blood from the open vessels.
* Muscles @ LS: * relatively weaker layer of muscle * less blood supply, compared to the upper segment.* Caesarean operation is done in the lower segment (across the top of the pubic hairline) * less likely to cause excessive bleeding.
* The LUS is an important anatomical part of the uterus* It has anatomical and functional aspects that can be utilized to produce a better outcome in pregnancy and labour* Attachment of the placenta to the LUS leads to a risk of bleeding 12 03/23/12
* Obstetrics by Ten Teachers* http://www.pregnancy-labour.info/2009/04/changes-in-uteru 13 03/23/12