Review Note
Last Update: 03/02/2025 07:48 PM
Current Deck: ACG Part 2::Obstetrics
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Commit #293135
SS_OB 1.20 Describe the mechanisms and progress of normal labour
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Commit #293135Labour = the process by which a fetus of viable age is expelled from the uterus, 3 stages:
- First stage = Onset of labour to full dilation of cervix
- Latent - mild, irregular contractions
- Active - 3-4cm -> 10cm
- Second stage = full cervical dilation -> expulsion of baby
- Third stage = Birth of baby to expulsion of placenta, umbilical cord and membranes
Mechanisms of normal labour:
- Descent of fetus: presenting part (ideally cephalic, occiput anterior (OA))
- Engagement of fetal head: can occur weeks prior to labour in primips
- Descent of fetal head
- Flexion of fetal neck: Through pelvic inlet
- Internal rotation of fetal head: To pass ischial spines at pelvic outlet, shoulders still transverse
- Crowning of head
- Extension of fetal neck: Chin delivered
- External rotation of head: Head lines back up with shoulders
Hormones involved:
- Progesterone: Made by corpus luteum, then placenta, inhibitory effect on contractile proteins and reduces oxytocin release and receptors, reduces prostaglandin production. Number or progesterone receptors reduces with on-going gestation
- Oestrogen: opposes progesterone - increases prostaglandin production, oxytocin receptor concentration and uterine contractility, gradual increase in third trimester
- Oxytocin: Stored in posterior pituitary , induces contraction and increases strength and frequency of contractions, also stimulates protaglandin synthesis
- Prostaglandins: cervical ripening and myometrial contractility, foetal membranes and decidua product PGE2, PGF2alpha