Intrauterine Pressure During the First Stage of Labour

Intrauterine pressure (IUP) is generated by contraction of the myometrium and can be measured directly with an intrauterine pressure catheter (IUPC) or estimated using Montevideo units. It varies through the course of labour, being lowest in early labour and rising progressively as labour advances.

Analysing the Options

• 40-50 mm of Hg (correct): During the first stage of established/active labour, uterine contractions generate intrauterine pressures in the range of 40-50 mm Hg above the resting baseline. This is the typical peak contraction pressure recorded with intrauterine pressure catheters during normal, effective labour.
• 2-3 mm of Hg: This value is far too low for an active labour contraction; it approximates the very low baseline tone seen in a completely quiescent, non-labouring uterus, not the pressure generated during the first stage of labour.
• 8-10 mm of Hg: This corresponds roughly to the uterine resting tone between contractions during labour, not the pressure achieved at the peak of a contraction.
• 100-120 mm of Hg: This figure is too high for spontaneous labour contractions of the first stage; such pressures are not physiologically generated by normal uterine activity and do not correspond to standard labour physiology values.

Conclusion

During the first stage of labour, effective uterine contractions raise the intrauterine pressure up to 40-50 mm of Hg, which is essential for progressive cervical dilatation. This is distinct from the low resting tone (8-10 mm Hg) maintained between contractions.