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As you might have picked up from previous pregnancy-related posts on the site (here and here), my wife and I have been expecting our first child. During labour and birth, terms like ‘epidural’, ‘gas and air’ and ‘induction of labour’ get thrown around, but what specific drugs do these involve? How do they work? What are the benefits and risks? This graphic and post aim to provide the chemical answers.

Some women choose to give birth without the administration of any drugs. Others choose to use medication to ease pain or to get labour started. Others still may end up requiring medication due to pre-existing conditions or concerns for the health of mother or child.

However a woman chooses to give birth, chemistry plays a part. We’ve already looked at how the balance of hormones in a woman’s body adjusts during pregnancy. These natural hormonal changes prepare the body for labour. In particular, oxytocin levels in the body rise when labour starts, triggering contractions.

Only around 5% of women give birth bang on their due date, but about 60% of women give birth on or before it. 35% of women will go into spontaneous labour within two weeks of their due date. If this does not happen, midwives and doctors may offer induction of labour.

Induction of labour kickstarts the process of birth. It’s commonly used when a baby is overdue or there are risks to the health of the mother or baby. It involves the insertion of a pessary, tablet or gel into the woman’s vagina. All three of these contain prostaglandins, usually a specific prostaglandin called dinoprostone.

Prostaglandins are produced naturally in the body. They’re made at the site of injury or infection, where they produce inflammation and pain, contributing to the healing process. In the case of induction, they help with the ‘ripening’ of the cervix. During pregnancy the cervix is firm and holds the baby in the uterus, but in advance of birth it softens and begins to open up.

Use of these induction methods can be enough for a woman to go into labour. If not, the woman’s cervix will eventually dilate sufficiently so that a midwife can manually break the waters. This can take up to 72 hours, so it’s by no means a rapid process!

Once a woman’s waters have broken, the pressure of the baby’s head with the top of the cervix triggers the release of oxytocin. As mentioned previously, this is the hormone that really gets the contractions going. For some women, contractions don’t always start when their waters break or aren’t long enough or strong enough.

In these cases, further medical assistance can be provided in the form of an oxytocin drip. A chemically identical synthetic version of oxytocin (often under a drug brand name such as syntocinon) is given to the woman. This increases the frequency and strength of contractions.

Later, after the baby has been delivered, a combination of oxytocin and ergometrine (known as syntometrine) can be given as an injection. This is to help with the delivery of the placenta. There’s evidence that shows that the injection of oxytocin at this point can reduce the risk of severe blood loss for the mother.

Once labour is in process, some women may choose to proceed without any pain relief. It’s worth noting that, if a woman’s labour has had to be induced as described above, it can often be more painful than labour that starts on its own. If they do opt for pain relief, there are several options available.

Most hospital birth centres and delivery wards will have a supply of ‘gas and air’ available. This name is somewhat ambiguous from a chemical perspective; gas and air is actually a 50:50 mix of oxygen and nitrous oxide. It’s inhaled during contractions, and while it doesn’t remove the pain, it can make it more bearable. It also makes you woozy and light-headed when you inhale it. This can lead to some entertaining outbursts during labour!

For more pain relief than gas and air can provide, opioid injections are given. Pethidine and diamorphine are commonly used. Again, they don’t completely eliminate pain but can make it a lot more bearable. Unlike gas and air, opioids can have side effects, including drowsiness, nausea and vomiting. A recent review of opioid use in labour found no clear evidence of adverse effects on newborn babies.

For further pain relief, a woman can request an epidural. An epidural is an injection of drugs around the nerves of the spine that carry pain signals from the uterus. Usually, the drugs administered are a mix of a local anaesthetic, such as bupivacaine, and an opioid, such as fentanyl. Though it can take up to 40 minutes for the epidural to kick in, once it does pain is significantly reduced.

As the drugs are injected into the back through a catheter, the pain relief drugs can be topped up using a button. The anaesthetist giving the epidural will test its effectiveness using a cold spray on the woman’s legs and torso, to gauge loss of sensation. Some women may experience numbness in the legs depending on the level of the anaesthetic administered. Most epidurals proceed smoothly, but there are some possible side effects.

In uncommon cases, epidurals may not work as effectively as desired. If a woman is still experiencing pain after the epidural, alternative pain relief methods may be offered. A spinal block may be one of these options.

Unlike the epidural, the spinal block involves the direct injection of a smaller amount of anaesthetic into the spinal fluid. This completely removes the sensation and ability to move below the level of the injection. Spinal blocks are sometimes used for births by Caesarean section or during more complex assisted deliveries. For emergency Caesarean deliveries, general anaesthetic may also be used.

My wife got to experience the majority of the above medications during the delivery of our son a few weeks ago, heroically making it through 40 hours from induction to birth. Happily, he arrived safely, and both he and my wife are doing well!

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References & Further Reading