A while ago a reader, Alice, asked what the -3 measurement related to when midwives* describe baby’s position and so, hopefully, I’m just about to explain the concept of assessing descent of the presenting part of baby, generally the presenting part being it’s head. Any midwives etc. reading this may well have apolexy reading my explanations which will be accuate, but not couched in medical terminology, also I have personally illustrated what I’m talking about but I am by no means an artist!
There are two ways that midwives estimate how far down baby’s head has gone. The first is how it’s done in both in pregnancy and during labour and is discovered by abdominal palpation, or having a feel of the woman’s bump. When the midwife concentrates her hands just above the pubic area, and possibly asks the woman to breathe in and then relax as she exhales, the midwife is trying to determine how much of baby’s head she can feel. This may be slightly uncomfortable as the lower part of the uterus, bump, can be a bit tender toward the end of pregnancy. The midwife will then record her findings in terms of fifths. If she can still feel all of baby’s head then she will write 5/5ths palpable (palp), look at my drawings below and the 5 drawings on the left side illustrate the gradual descent into the pelvis as felt abdominally. Basically, the less of the head felt the the lower the number of fifths palpable. Sometimes a midwife will write ‘Engaged’ (eng) rather than a fraction, when this is writen it means that, in her estimation, the widest part of baby’s head has gone through the brim of the pelvis. (In the photo the brim is the top of the inner circle).
Below is a photo of a female pelvis. The angle is such that if this were a real woman the photographer would be standing at the woman’s feet whilst the woman was lying on her back with her bottom tilted upwards.If you look at the inner circle of the pelvis you can see that I have stuck blue stars on little bony protruberances, these little lumps are called the ischial spines, the gap between them is about 10.5- 11 cms, and these are the landmarks that an estimation of descent, or station, of the presenting part is based upon when an internal examination is performed.
S0, the spines are 0 (nought) in a midwife’s world. When a vaginal examination is performed the midwife will hope to be able to feel these small bony protruberances and then note where baby’s head is in relation to them. The spines are 0 and whether the head is above or below is expressed in centimetres, minus ( – ) if it is still above, plus ( + ) if below or, if it is level with them, then a midwife will often write ‘at spines’. My drawing shows the gradual descent through the pelvis, sort of!
This is a simplistic explanation of how estimations of descent are conducted and expressed, below are a couple of on-line resources which are well worth reading.
* I’ve written ‘midwife’. It could be a G.P or obstetrician.