3 - Labour

Question 1

Summarise gestational age timings

Answer 1

GESTATIONAL AGE TIMINGS

Term - 37-41 weeks

Post-term - 42 weeks or more

Pre-term - 22-37 weeks

Extremely preterm - 22-28 weeks

Very preterm - 28-32 weeks

Moderate to late preterm - 32-36 weeks

Miscarriage - Less than 22 weeks (non viable infant delivered).

Early miscarriage - First trimester

Late miscarriage - Second trimester less than 22 weeks.

Question 2

How many normal term births occur in the UK per year?

Answer 2

NORMAL TERM BIRTHS

Around 700,000 per year in the UK

[does not include caesarean section births]

Question 3

How many preterm labour births occur in the UK per year?

Answer 3

PRETERM LABOUR BIRTHS

Around 45,000 per year

Question 4

How many extremely preterm births occur in the UK per year?

Answer 4

EXTREMELY PRETERM LABOUR BIRTHS

Around 4,500 per year

Question 5

What is the estimate for incidence of early miscarriage per year in the UK?

Answer 5

The incidence of early miscarriage in the UK is thought to be around 35,000 per year

Question 6

Define ‘human labour’

Answer 6

The process of expulsion of the fetus and the placenta from the uterus.

Question 7

List the stages of human labour as per a definition

Answer 7

The stages of labor include:

• first stage, beginning with the onset of uterine contractions through the period of dilation of the os uteri
• second stage, the period of expulsive effort, beginning with complete dilation of the cervix and ending with expulsion of the infant
• third stage or placental stage, the period beginning at the expulsion of the infant and ending with the completed expulsion of the placenta and membranes

Question 8

Why can labour take a long time?

Answer 8

This is mainly because the uterus and cervix, which are the main tissues involved in labour, have to undergo a substantial change in structure and functions as they transition from what is needed for pregnancy (in which state they have been for 9 months) to what is needed for the delivery of the infant.

Question 9

What is another word for contraction of the uterus?

Answer 9

Involution of the uterus

Question 10

Summarise the main tissues in human labour

Answer 10

NOTE: the cervix is tightly closed

Question 11

List the main events of human labour

Answer 11

Cervical ripening and effacement

Co-ordinated myometrial contractions, Preceded by “Braxton Hicks contractions” or ‘contractures’

Rupture of fetal membranes

Delivery of infant

Delivery of placenta

Contraction of uterus to limit maternal blood loss

Question 12

What occurs in the first stage of labour?

Answer 12

FIRST STAGE OF LABOUR

Changes in the cervix, and in the myometrium

Rupture of the fetal membranes also normally occurs during the first stage.

This stage is usually the longest lasting for ~8 hours – this is likely to be longest in a woman having her first baby, and shorter in subsequent pregnancies.

Note that this stage is very variable, with both shorter and longer times being quite common.

Question 13

What occurs in the second stage of labour?

Answer 13

SECOND STAGE OF LABOUR

Delivery of the infant is the second stage of labour

This usually last about 30 minutes, but again can be longer in a first pregnancy and shorter in following pregnancies.

Question 14

What occurs in the third stage of labour?

Answer 14

THIRD STAGE OF LABOUR

Delivery of the placenta, which should occur within 30 minutes of delivery of the infant.

Delivery of the placenta is associated with very powerful contractions of the uterus, leading to a rapid decrease in overall size – this is referred to as involution.

Once the placenta has been delivered, this involution of the uterus is very important, as this is the primary process through which blood flow through the spiral arteries is stopped.

This process is linked to increased maternal levels of oxytocin – if it does not occur spontaneously, an injection of oxytocin (or similar muscle contracting agent) can be given to accelerate the process.

Question 15

Summarise the main stages of human parturition

Answer 15

Question 16

Summarise cervical ripening and dilatation

Answer 16

Ripening = softer and flexible

Dilatation = thinner and stretched sideways

This requires extensive remodelling of the extracellular matrix of the cervix, a process that can take many hours.

This process is also accelerated by the increasing pressure of the fetal head on the cervix, caused by the increasing strength and decreasing gaps between myometrial contractions.

Question 17

During labour, what mediators regulate changes in the cervical, myometrium and fetal membranes?

Answer 17

CERVIX

Prostaglandin E2, interleukin-8, Matrix metalloproteinases (MMPs)

MYOMETRIUM

Prostaglandin F2α (E2) levels increased from fetal membranes

Oxytocin receptor increased

Contraction associated proteins increased

FETAL MEMBRANES

Inflammatory process in fetal membranes

Prostaglandins, interleukins, MMPs

These changes may generally be described as ‘inflammatory’, as they involve molecules that are present during inflammatory processes anywhere in the human body.

It seems that similar mechanisms are involved in labour at all gestational ages, so the changes summarised above will be present during preterm labour at 28 weeks, and during term labour at 40 weeks of gestational age.

Question 18

What percentage of human pregnancies are delivered term?

Answer 18

90% of human pregnancies are delivered within the expected time-frame, usually referred to as ‘term’ or 37-41 weeks of gestational age.

Infants delivered earlier than this are at increased risk of a range of complications.

Question 19

What maternal risks exist in pregnancy and labour?

Answer 19

MATERNAL RISKS

Pregnancy itself = relatively low risk to maternal health, but there are exceptions

• If the mother is in poor health at the beginning of pregnancy (e.g. due to socio-economic factors such a poor nutrition), then the additional strain on her system caused by pregnancy may have considerable impact.
• Pre-eclampsia affects the maternal vascular system, which can pose risks to the health of the mother.

Main risk to the mother = labour itself

• particularly the necessity for uterine involution after delivery of the placenta.
• Due to the remodelling of the spiral arteries, they cannot vasoconstrict in the normal way to decrease blood flow, and hence blood loss into the uterine lumen.
• The powerful uterine contractions of involution effectively close the spiral arteries, and limit blood loss. It has been estimated that 1%-3% of women died from complications of pregnancy during pre-medical centuries – and this refers to each pregnancy, so with large numbers of serial conceptions, overall maternal death rates could be high (10 pregnancies, with a 2% risk of peripartum in each = 20% maternal death rate).

Preterm labour and delivery does not pose any additional risk to the mother; the smaller size of the infant in preterm deliveries may lead to less affect on maternal tissues.

Question 20

What fetal risks exist in pregnancy and labour?

Answer 20

FETAL RISKS

Delivery at term normally has few specific risks for the infant, but delivery at gestations less than 32 weeks are much more likely to cause infant morbidities or mortality.

Data from one study serves to emphasise the main points: Survival increased rapidly with increasing gestational age so that by 26 completed weeks of pregnancy, nearly 80% of babies left hospital alive. The great majority of these infants had major morbidities (87%), so that their quality of life, and life expectancy will be severely affected. Ongoing medical support may well be needed.

In addition to the risks of early delivery, linked to incomplete development of lungs, brain, digestive or immune systems, it seems that the fetal brain is particularly sensitive to inflammatory mediators. As labour is an inflammatory process, and one clear cause of preterm labour is intrauterine infection, the incidence and severity of brain damage is particularly high in these extremely preterm infants.

Question 21

What complications are extremely preterm labour cases linked to?

Answer 21

Many of these extremely preterm labour cases are linked to intrauterine infection, or other uterine complications such as bleeding, but not all causes have been identified.