What causes labour?

The exact mechanisms causing labour in pregnant mothers are uncertain. However, it is very likely that there is a series of events involving various hormones, genes and other substances within your body, resulting in the dual process of contractions in the womb and opening of the neck of the womb (cervix). It is also believed that this is initiated by the baby and the placenta.

What is induction of labour (IOL)?

You may have heard this term many times, but do you know what it means? Induction of labour (IOL) is any medical intervention performed that stimulates the onset of labour pains (i.e. to establish labour), aiming to result in the delivery of the baby vaginally (read the article on Stages of Labour).

Do I really need to undergo IOL?

A variety of medical conditions may arise during the course of your pregnancy that may put you or your baby’s well-being at risk. This may necessitate your doctor offering you an early delivery of your baby. In certain instances where you or your baby is assessed to be unable to tolerate the stress of labour, a cesarean section may be suggested to expedite your delivery instead. Only if time permits and there is no immediate danger to you or your baby, an IOL can then be offered.

What are the medical reasons that may necessitate IOL?

These can be broadly categorized into conditions that can put either you or your baby at risk should the pregnancy be allowed to progress. Under such a circumstance, your doctor would have investigated you thoroughly and assessed that it would be safer for either mother or baby that the delivery occurs before your due date or in rarer cases, even before maturity is reached.

  • Pre-eclampsia (PE) is a serious medical disorder caused by pregnancy. PE may affect many organs but commonly manifests as high blood pressure, leaking of protein in the mother’s urine and generalized swelling in the mother (read the article on Pregnancy Complications). Delivery of the baby will treat PE and reduce the risk of harm. Hence IOL is commonly performed before the mother reaches the severe stage of PE.

  • Diabetic mothers are commonly induced before their estimated delivery date. Babies of diabetic mothers are commonly larger and early delivery makes it less challenging. Babies of diabetic mothers are also at increased risk of stillbirth, if not delivered by the due date (especially if the sugar control is poor).

  • Intrauterine growth restriction (IUGR) means that the baby is not growing to his/her full potential (read the article on Pregnancy Complications). It is preferable to deliver the baby and provide nutrition externally.

  • Intrahepatic cholestasis of pregnancy (ICP) is a condition which presents with generalized skin itch in the mother. There is also evidence of liver dysfunction. ICP is associated with stillbirth and IOL at 37–38 weeks is often recommended to prevent this.

  • Women with a personal history of precipitous labour (i.e. very rapid delivery) are commonly offered IOL after 37 weeks of gestation for practical reasons. In subsequent labour, their risk of rapid delivery remains high and the time from onset of pain to expulsion of baby may be less than 60 minutes. Hence, IOL offers certainty of professional assistance.

  • Post-date is a condition whereby your pregnancy goes past the expected due date. An IOL may then be offered. Studies have indicated that an IOL under these circumstances would reduce the incidence of stillbirth and the need to perform a cesarean section for fetal distress during labour.

Are there any non-medical reasons for IOL?

  • Social request for personal reasons — Patients often request for IOL. The reasons include fatigue, sleeplessness, and muscular discomfort as the pregnancy advances, personal choice of delivery date, choice of date after consulting the horoscope and fear of stillbirth (these patients commonly have close friend/relatives with unfortunate occurrences).

  • Favourable cervix — Some patients are offered IOL when they are deemed “ready to give birth” by their doctor. This commonly involves a routine antenatal consult with their doctor near the delivery date. The doctor performs a vaginal examination and informs the patient that her cervix is dilated and she is ready. This offer is common in modern obstetrical practice. Firstly, as the cervix is ready, the chance of successful IOL resulting in a vaginal birth is good. While the occurrence of stillbirth at term is rare, the consequence is catastrophic. A significant proportion of mature stillbirths remain unexplained. Hence, IOL with consequent delivery allows us to reduce the risk of this occurring.

It is important to note that routine IOL in uncomplicated pregnancies has not been successful in reducing stillbirth rate and results in higher rates of forceps, vacuum and cesarean deliveries.

Are there any risks to an IOL?

As with any procedure, there are certain concerns associated with an IOL. They include:

  • Hyper-stimulation of the womb — Overly frequent contractions that result from the IOL may reduce the oxygen flow to the baby which manifests as a drop of baby’s heartbeat. Some medications may then be administered. However, in unresponsive cases, an emergency cesarean section may be performed.

  • Uterine rupture — Another potential serious complication is rupture of the womb with expulsion of the baby into the mother’s abdomen. This can result in a stillborn or permanent damage to the baby’s brain. Fortunately, this is a very rare complication of IOL especially in modern obstetrics. The main risk factors for rupture are in patients with one or more previous cesarean sections and patients who had more than five previous vaginal deliveries (i.e. grand multiparity).

  • Failure of an IOL — In some instances, the cervix (neck of the womb) remains unresponsive to repeated courses of prostaglandin, and remains tightly closed. In other instances, the labour does not result in progressive dilatation of the cervix. The baby’s head may not descend low enough for safe vaginal birth. In rare occasions, the baby is very sensitive to the agents (prostaglandin and oxytocin) used in IOL and responds by manifesting in unreassuring fetal heart trace. In this case, the infusion has to be reduced or stopped, which consequently may be insufficient to allow adequate power for vaginal birth. When IOL fails, a cesarean section (along with the accompanying surgical risks) is performed to deliver the baby.

As such, an IOL is only performed when the benefits of a delivery outweigh the above-mentioned risks or when your obstetrician is confident that the risks associated with IOL can be adequately minimised by appropriate precautionary measures.

When am I considered unsuitable for an IOL?

IOL is not performed when you are deemed unsuitable for a vaginal delivery in the first place. They include conditions such as a low-lying placenta, breech or transverse lie of the baby.

If you have had one previous cesarean section, an IOL is generally not advised as the risk of a uterine rupture is approximately 2.5%. This is five times higher than the risk of uterine rupture should you go into spontaneous labour on your own, and is considered to be unacceptable by many.

How is an IOL performed?

Labour starts when the cervix initially soften, shortens and dilates (read the article on Labour Pain Relief). This can be achieved through the insertion of prostaglandin, a hormone, into the vagina. Locally, the pessary known as Prostin is commonly used. Once the cervix is adequately dilated and effaced (thinned out), the membranes can be ruptured and an oxytocin infusion (another hormone) can be started to maintain the labour contractions.

Prostaglandin application vaginally

  • Prostaglandin causes softening of the cervix (neck of womb) through a disaggregation of collagen fibers in the cervix. In addition, prostaglandin stimulates uterine muscle activity leading to labour. It is administered vaginally and may be in gel or pessary. Prostaglandin induces the onset of painful uterine contractions which may lead to the opening of the softened cervix. Once the prostaglandin is inserted, the patient is required to stay in hospital for monitoring. The frequency of uterine activity and baby’s heart rate pattern are observed.

  • Women respond differently in terms of speed. The lowest dose regime is commonly employed to prevent over-stimulation. The patient with a favourable cervix has a better chance of responding more quickly. A favourable cervix (determined by the doctor by vaginal examination) is soft, effaced (thin), dilated, faces to the front, with the baby’s head well applied and low in pelvis. Some patients may respond with establishment of regular labour pains within six hours; while others may take up to 2–3 days.

Rupture of membranes and oxytocin infusion

  • Once the cervix is favourable, the doctor may rupture your membranes (ROM) followed by administering an oxytocin infusion. Rupturing of membranes involves using an amniohook (specially designed instrument) to break the waterbag. ROM alone will induce painful contractions in a proportion of patients. The frequencies of contractions are monitored in the delivery suite and if this is inadequate, an infusion of oxytocin is given to the blood stream via an intravenous drip.

  • Oxytocin is a naturally occurring hormone produced by the brain that stimulates the womb. This aims to keep the contraction frequency to about four in every 10 minutes. Unlike prostaglandin, oxytocin infusion can be stopped by switching off the infusion. As oxytocin has a short half-life in the mother’s blood, the concentration reduces rapidly and this averts potential over-stimulation.

Myths surrounding induction of labour (IOL)

Myth 1: The patient suffers some pain from the time the prostaglandin is introduced

This is not necessarily true. Some patients may experience mild pain while the cervix is responding slowly to the prostaglandin. In this instance, she is encouraged to ambulate.

Myth 2: IOL is ‘more painful’ than normal labour

There is no scientific evidence to support this myth. One reason for this perception is that a successful IOL will bring on the labour pains. The inevitable negative association leads to the negative perception.

Myth 3: IOL is unnatural

There is nothing unnatural about going through a labour brought about by an IOL. Once labour is established (read the article on
Stages of Labour), the same rules on progress of labour apply.

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By Dr TAN Thiam Chye, Dr TAN Kim Teng, Dr TAN Heng Hao, Dr TEE Chee Seng John,
KK Women’s and Children’s Hospital

The New Art and Science of Pregnancy and Childbirth, World Scientific 2008.

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