Post-Term Pregnancy and Labour Induction

There are two definitions of post-term pregnancy :

a) From WHO and FIGO : pregnancy which has complete a total of 42 weeks or 294 days, starting from the first day of LMP

b) From American O&G Association : period of gestation of 2 weeks beyond estimated date of delivery


Complications 

1) As gestational age increases, the incidence of still birth increases.

2) Placental dysfunction, which leads to reduced fetal renal perfusion, and consequently oligohydramnios. With a reduced amniotic fluid volume, there's increased risk of cord compression, which may eventually results in fetal deccelerations.

3) One of the sign of increasing fetal maturity in late pregnancy is meconium passage. Since it's accompanied by reduced amniotic fluid volume, the meconium passed-out is not diluted. Hence, under such circumstances, there's increased risk of meconium aspiration.

4) Fetus continues to grow after 40 weeks. Hence, in post-term pregnancy, macrosomia may be the outcome. There's significant increased risk of birth asyphxia and trauma as a result of macrosomia.

Management

1) Accurate pregnancy dating

Determination of uterine size via clinical examination is more accurate during early pregnancy than late.

<12 week of pregnancy, CRL measurement is most accurate in dating.
12-20th week of pregnancy, HC, FL and BPD is used for pregnancy dating.

Studies shown that increased early USG scan reduces the incidence of labour.
induction of "post-term pregnancy". 

However, after 20th week of pregnancy, USG dating is no more accurate.

2) Labour induction

Labour is induced at 41 week of gestation or beyond. Pre-induction cervical ripening increases the success rate of induction.

OR

3) Expectant management

This is a wait-and-see policy, done together with continuous fetal well-being assessment, using :

a) Cardiotocography
b) Biophysical profile
c) Fetal movement count
d) Amniotic fluid index
e) Doppler's velocimetry

Studies shown that labour induction has a lower perinatal mortality and morbidity rate, and lower rate of C-section as compared to management of post-term pregnancy expectantly.

Induction

Defined as initiation of uterine contraction in non-labour mother, to achieve vaginal delivery.

Indications

a) Urgent

Severe antepartum haemorrhage
Severe proteinuric hypertension (pre-eclampsia)
Suspected fetal demise (suspicious CTG and oligohydramnios)
Severe IUGR
Severe maternal disease not responsive towards treatment
Chorioamnionitis

b) Semi-urgent

Poorly controlled diabetes
IUGR without fetal compromise
Prelabour rupture of membrane at term
Isoimmune disease near or at term

c) Elective

Gestational diabetes mellitus
Intrauterine fetal demise
Suspected macrosomia
Prolonged pregnancy
Logistic causes

Contraindications

Placenta praevia
Invasive cervical carcinoma
Previous scar of classical C-sec with vertical incision
Abnormal lie (transverse/oblique)
Pelvic structural abnormalities
Active genital herpes
Uterine surgery of full wall thickness

Various methods in labour induction

a) Cervical assessment

To assess whether the cervix is favourable for induction or not, a modified bishop's score is used


Nulliparous/multigravida with favourable cervix

1mg followed by 1mg 6 hrly with max dose of 3 mg

b) Misoprostol

Misoprostol are as beneficial as prostaglandins, and they are cheaper and storage is easier.
However, risk of uterine hyperstimulation is higher, hence it's contraindicated in women with previous uterine surgery.

c) Mechanical methods

Using foley's catheter

First, clean the vaginal canal.
A foley's catheter is inserted into endocervix beyond cervical os, then 30-50ml of saline is infused to fix the catheter into position.
Catheter is kept for 24 hrs or until spontaneous expulsion.
Should any undesirable effects occurs, it can be easily removed.

Sweeping of membranes

This method can be employed in low risk women at term. Finger of examiner is inserted beyond the cervical os, and the membranes are detached from the lower uterine segment.

By unknown mechanism, a cascade of event is triggered and labour can be induced.

Amniotomy

Amniotomy or artificial rupture of membrane can be done in women with favourable cervix. A forewater amniotomy is usually done, by using the amnihooks available in most of the labour ward.

The amount and colour of amniotic fluid is recorded.

Labour usually starts 6-12 hours after amniotomy is done, in most cases.
However, the most important drawback of amniotomy is, the induction-delivery interval can be occasionally unpredictable.

Hence, currently, a dose of oxytocin infusion is given after amniotomy to reduce the induction-delivery interval.

Oxytocin

To purpose of giving oxytocin is to :

a) Sufficient cervical changes take place
b) Ensuring fetal descent
c) At the same time, prevent uterine hyperstimulation

The dose used for initiation is greater than the maintainance dose.
The usual approach is to escalate the doses until a desirable frequency of contraction (4-5 contractions in 10 minutes, each contraction > 40 sec) is achieved, then maintain the dose until delivery.

Firstly, titrate oxytocin at 4.5-5 mu/min to 14-15 mu/min until desirable frequency of contraction is achieved.
If desirable frequency of contraction is not achieved, increase the dosage by 4-5 mu/min every 30 mins.

Since the interval of dose delivery is important, hence an electronic based gravity fed system of delivery is more reliable than those manual pumps.

Risks : Uterine hypertonus -> Uterine rupture or uterine scar dehisicience
        Fetal hypoxia
        Water retention -> to be careful in mothers with cardiac diseases and PIH
      
Prolonged infusion -> Maternal hyponatremia (headahce, nausea, psychosis, seizures and Fetal hyponatremia (apnoeic and cyanotic attacks)

Any failed induction is equivalent to conduct a LSCS.
Uterine hyperstimulation : 2 min or greater duration of each contraction and frequency >5 in 10 mins.

Category: Obstetrics Notes