Lower uterine segment

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an overview of the uterine anatomy with clinical implications with regards to placenta praevia

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  • 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

    1. 1. NurFakhriyyah, LokemanHazli, Alia NadhirahAssociate Professor Dr Hanifullah Khan 1 03/23/12
    2. 2. * 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
    3. 3. * 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
    4. 4. * 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
    5. 5. * 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
    6. 6. 6 03/23/12
    7. 7. * 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
    8. 8. * 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.
    9. 9. 10 03/23/12
    10. 10. * 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.
    11. 11. * 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
    12. 12. * Obstetrics by Ten Teachers* http://www.pregnancy-labour.info/2009/04/changes-in-uteru 13 03/23/12

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