Induction and Augmentation of Labour
last authored: Oct 2011, David LaPierre
last reviewed: Oct 2011, Rhonda Verleye
Introduction
Normally, signals from the uterus and fetus such as prostaglandins help to trigger labour. However, there are various situations in which labour must artifically be initiated. This process is termed 'induction'. Cervical ripening is the treatment of the cervix with medications such as prostaglandins or physical means such as a Foley catheter to increase the likelihood of successful vaginal delivery prior to induction.
In other situations, a women in labour requires assistance with the frequency or strength of her contractions; this assistance is termed 'augmentation.' This can include simply rupturing the membranes or by use of oxytocin.
Indications for Induction
Induction may be considered when the risk of ongoing pregnancy is greater than the risk of induction, labour, and delivery. It is critical to discuss potential benefits and risks with the parents.
Post-term pregnancy, or past 42 weeks gestation, is one of the most common reasons for induction. Post-term pregnancy is associated with increased risk of morbidity, mortality, and need for instrumentation. The Society of Obstetricians and Surgeons of Canada suggests induction after 41 weeks.
Conditions that carry a high risk, and therefore are viewed as clear reasons for induction, include:
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Other indications include:
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Not acceptable indications include:
- suspected macrosomia
- healthcare provider or patient convenience or preference
Risks and Contraindications of Induction
Induction done properly, and for the right reasons, is beneficial. However, it is not without risk, and complications can include:
- failure to achieve labour
- increased risk of instrumentation and cesarean section (especially in nulliparous women)
- uterine hyperstimulation and fetal compromise
- uterine rupture
- chorioamnionitis
- cord prolapse (if amniotomy is carried out)
- preterm infant (if dates are incorrect)
Reasons for avoiding induction (and for strong consideration for Caesarean section) include:
- placenta previa, vasa previa
- fetal lie or presentation not compatible with vaginal delivery (ie transverse or footling)
- prior C-section with classical or inverted T incision
- significal prior uterine surgery
- active genital herpes
- known pelvis incompatible with vaginal delivery
- previous uterine rupture
- invasive cervical cancer
Avoiding Induction
Given the risks, induction should be avoided as possible.
As the most common reason is for post-term pregnancy, effective dating is absolutely critical. This is a strong reason for early ultrasound.
Sweeping the membranes, also known as stripping, is a practice done during prenatal visits in which a sterile gloved finger is used to dilate the cervix and separate the amniontic membranes from the uterus. This appears to reduce the likelihood of post-dates pregnancy via production of prostaglandins, with the number needed to treat of 8 (Boulvain, Stan, and Irion, 2005). However, sweeping can be uncomfortable and cause minor bleeding and irregular contractions.
Assessment of Cervix
The most favourable indicator is the state of the cervix, often as assessed through the modified Bishop score; the higher the score, the more likely success with induction will be. Cervical dilation appears to be the most valuable factor.
The modified Bishop score is as follows
0
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1 |
2 |
3 |
|
dilation (cm) |
0 |
1-2 |
3-4 |
>5 |
effacement % cm length |
0-30 >3 |
40-50 1-3 |
60-70 <1 |
>80
|
consistency |
firm |
medium |
soft |
|
position |
posterior |
mid |
anterior |
|
station |
Sp -3 or above |
Sp -2 |
Sp -1 or 0
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Sp +1 or lower |
Induction - Unfavourable Cervix
If the modified Bishop score is <6, it is deemed to be 'unfavourable'. Cervical ripening is required prior to induction.
The cervix should be prepared prior to induction in order to maximize chances of success. Options include:
- catheterization
- medications
Balloon catheterization
Cervical ripening may be attempted using a 14-18 F Foley with a balloon.
Procedure
The Foley should be tested beforehand. Using sterile technique, insert the balloon paster the cervical os. Inflate with 30-80 cc of water. The balloon typically falls out once the cervix is 4 cm dilated.
Maternal infection appears increased with Foley catheterization.
Medications
Some centres use prostaglandins on an outpatient basis, though usually require prior fetal surveillance. Benefits are decreased duration of labour and decreased need for oxytocin.
prostaglandin E2 (dinoprostone)
PGE2 derivatives can be used as vaginal or intracervical gels. It is important not to mix these, as the dose will be wrong.
They should not be used for augmentation of labour and should be used cautiously with oxytocin in previous C-section.
Monitor FHR for 1-2 hours. The main risk is for hyperstimulation; gel preparations can cause complications as they are difficult to remove.
prostaglandin E1 (misoprostol)
PGE1 tablets may be taken orally or vaginally. Its use requires close monitoring.
PGE1 carries a risk of hyperstimulation; avoid in previous C/S or uterine scar due to risk of uterine rupture.
Also, avoid oxytoxin within 4 hours of misoprostol.
Induction - Favourable Cervix
If the Bishop score is greater than 6, cervical ripening is not required. Options for induction include:
- amniotomy
- oxytocin
Amniotomy, or artifical rupture of membranes (ARM)
A hooklike instrument is used to rupture the amniotic sac. Note the amount, colour, and consistency of the fluid to assess for meconium, and assess fetal heart rate. Be cautious if presenting part is high due to risk of cord prolapse. Amniotomy may be combined with oxytocin for improved success with induction.
Oxytocin
Oxytocin, used for induction, is a synthetic version of a hormone naturally produced in the hypothalamus. Oxytocin receptors in the uterus increase in number as term approaches, and serum levels increase in spontaneous labour to increase smooth muscle contraction in the uterus.
It is normally given by IV infusion at a low rate (miliunits per minute, or mU/min).
Oxytoxcin is safe, though should only be used in settings where adquate staff and supplies are available to respond to dystocia and other emergencies. Always use electronic fetal monitoring with oxytocin.
Hyperstimulation may occur with oxytocin infusion. In this setting, stop oxytocin, treat the hyperstimulation, and consider restarting oxytocin at a lower dose. Hypotension is possible with bolus IV use, and water intoxication may also result from cross-reactivity with ADH receptors. Lastly, risk of postpartum hemorrhage is increased with use of oxytocin for induction; prepare for it.
Hyperstimulation
D/c oxytocin or remove prostaglandin
Intrauterine rescitation
Tocolytics: nitroglycerin IV push.
Augmentation Protocols
If indicated, oxytocin may be used to increase the strength and frequency of contractions. Each woman responds differently to oxytocin, and response can also differ throughout labour. Most providers would suggest starting with a low dose and increasing it every 30 minutes. Low- and high-dose protocols may be used.
Ensure adequate time has elapsed before expecting results.
Many health care providers are concerned about oxytocin due to risk of fetal distress and uterine rupture. However, while hyperstimulation can reduce oxygen transport to fetus, correct dosing is helpful for strongly reducing these outcomes, which are caused by excessive contractions.
Other adverse consequences include:
- water intoxication (prevented by limiting free water)
- hypotension (avoided by not giving IV boluses)
Resources and References
Boulvain M, Stan C, Irion O. 2005. Membrane sweeping for induction of labour. Cochrane Reviews. CD000451.
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