Here are some reasons they might say you need a vaginal examination (VE), and the pros and cons of consenting or declining.
The disadvantages of VEs
They increase the risk of infection
They can be painful
They are subjective
They do not give accurate predictions about future progress overall
The information provided can influence morale of everyone present
They can lead to unnecessary intervention
To assess whether you are in spontaneous labour because we cant give a room to someone who may not give birth imminently
Active labour is typically defined as having 3-4 regular, strong contractions in 10 minutes, with 4cm dilation of a soft, central cervix . However, this is an arbitrary definition and vaginal examinations are notoriously subjective. They also tell you more what has happened, rather than what will happen and when.
You may need and are entitled to support before this, but the issue with rooms/staff is a real one. If you are contracting, need support, but dont seem to be in active labour (whether you had a VE or not), you could be admitted to the antenatal ward and have an opiate or gas and air plus some support from staff. You needing support is separate from you being in active labour.
Sometimes people want to use the pool early on in labour, this can be known to prolong the early stages of labour. However, we also recommend a bath and paracetemol in early labour to people on the phone, both of which have been known to delay progress. I recommend doing what you want because we dont know which person you will be unless we try and see something like a reduction in contraction intensity.
The plus side is that hearing that "the books" agree you are indeed in active labour as you suspected can be validating. You know you know your body.
To assess whether your labour is progressing
There are some increased risks when a labour goes on "too long" but we dont actually know what "too long" is. We dont really know how quickly someone should dilate. Plus, dilation isnt the only sign of progression.
If you ask someone in labour if they think things are moving along, they will say yes, no, or I dont know. If they say they dont know, you could ask them if they want you to feel their tummy or do a VE to check the babies position and descent as well as their dilation. We dont need to do all of these things, we can use some of them along with behaviour and the length and strength of contractioms to build a picture.
However, where there are already risks such as compromised baby or a mother at risk of haemorrhage, it can help us continually gauge the risk and recommend interventions that will minimise the chance of it occurring before it happens.
You want an intervention like an epidural or opiate
Sometimes, a sudden request for something like this might mean there is progress and the second stage of labour is rapidly approaching.
Research is patchy, but there are definitely cases where I feel like a "late" epidural led to intervention like forceps because of a lack of feeling, but it is never too late to have an epidural and labour could go on for 3 hours from 10cm dilated. This is where informed choice comes in. Pretty sure the latest reviews say epidurals do not impede labour.
Opiates are a bit different in that the baby can be affected after birth. The vast majority of affected babies are only sleepy and lazy feeders. However, if someone specifically wanted opiates AND it appeared labour might have progressed significantly, there is more clinical justification for a VE than there would be with someone wanting an epidural.
Its equally valuable to step up conservative support, recommend position changes, massage, warm or cold compresses, etc.
There has been a change in the baby's heart rate
Sometimes this can signify that there has been progress and the baby is just responding normally to being on its way out. Other times, it can indicate fetal distress. While other behaviours like an urge to push can make it obvious that progress has occurred, it is often a time where knowing the exact position of the baby can help make the safest decision for all.
This is a time where there is total clinical justification for strongly recommending a VE because it will give vital potentially life saving information.
You are being induced
Despite there being oral method of induction avaliable, a VE can tell you what methods are clinically indicated (some people are dilated enough to just have their waters broken). This can help you avoid unneccessary drugs and procedures and even stay in a low risk birth setting.
To check if an intervention has worked/if we need to do more
If you are being induced, or your spontanous labour stalled, staff often do something, like break your waters, and then check a couple of hours later to see if there has been progress.
Sometimes it is obvious there has been (see above) and the need to check with a VE (especially with no waters) becomes less clinically justifiable. Other times, it is not obvious and a hormone drip is the next line of action. If someone has had very few contractions and/or reports limited change in sensation, then you may as well just start the drip without the discomfort and infection risk of a VE.
While on the drip, having progress checks will give you information about whether it is likely to work. This is clinically valuable because the drip increases the risk of haemorrhage at the point of birth. You have to balance the risk of its long term use with that risk of bleeding.