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.
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.
