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Mechanics of Labour
The ability of the foetus to successfully negotate the pelvis is dependent on 3 variables – the 3 P’s.
‘POWERS’ (uterine contractonns
Uterine actvity is characterised by frequency, intensity, and duraton of contractons.
It can be externally measured by a tonometer or an IUPC/intrauterine pressure catheter.
It can also be assessed by measuring the rate of cervical dilaton and descent of presentng foetal part.
“Adequate” uterine actvity 3-5 strong contractons in 10 mins 150-200 montevideo units (via IUPC)
‘PASSENGER’ (foetuns
1) Foetal size
2) Foetal attute
- Heat feeion
- Heat eetension
3) Lie: relatonship between the long axis of fetus and long axis of the uterus
- Longitutinal
- Oblique
- Transverse
4) Presentaton: relatonship between the leading fetal part and the pelvic inlet. This efects the diameter to
pass through the pelvic inlet.
- Vertee (cephalic/head) - Head frst, where occiput is the
leading part. Ideal as this is the smallest diameter
(suboccipitobregmatc 9.5cm)
- Brow (cephalic/head) – Head frst, but head is slightly
extended rather than fexed. Widest diameter
(Mentovetcal 14cm)
- Facial (cephalic/head) - Head frst, but head is completely
extended rather than fexed
- Shoulter – Shoulder frst.
- Breech – Botom frst (3-5%)
5) Positon: relatonship of occiput (back of head) to maternal pelvis if
cephalic
- Ocipito-anterior/OA (“back to front”) – Head frst, facing
backwards (towards coccyx) allowing optmal head fexion. IDEAL
FOR DELIVERY.
- Ocipito-posterior/OP (“back to back”) – Head frst, facing forward
(towards pubis). Not ideal, but delivery should be atempted
before manipulaton.
- Ocipito-transverse/OT – Head frst, facing either lef or right.
Delivery not possible without manipulaton.
6) Staton: Relatonship of the bony presentng part of the fetus to the maternal
ischial spines.
- Zero staton (at the level of the spines) – usually at 1/5 th palpable
- +2 staton (passed spines by 2cm) etc.
7) Engagement: The fetus is engaget if the widest part (typically the widest
circumference of the head) has entered the inlet.
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