Efficacy of Bicarbonate on the Risk of Assisted Delivery in Nulliparous Women With Prolonged Labour

The goal of this clinical trial is to learn if oral bicarbonate can treat prolonged labour in nulliparous women with prolonged labour. The main question it aims to answer is: Do oral bicarbonate and restrictive oxytocin use reduce assisted delivery rate (emergency cesarean and vacuum/forceps delivery) in nulliparous women with prolonged labour. Researchers will compare bicarbonate and placebo (a look-alike substance that contains no drug) to see if bicarbonate works to treat prolonged labour. Participants will: * drink bicarbonate or placebo * have cervical dilatation reassessed after 2 hours. If still slow progression, they will follow standard protocol for oxytocin augmentation * will receive a questionnaire 1 month post partum to assess birth experience 1/3 of participants will have amniotic fluid lactate measured at inclusion and after 2 hours