Fetal head descent can be expressed as fetal station and engagement. Station is traditionally based on clinical vaginal examination of the distal part of the fetal skull and related to the level of the ischial spines. Engagement is based on a transabdominal examination of the proximal part of the fetal head above the pelvic inlet. Clinical examinations are subjective, and objective measurements of descent are warranted. Ultrasound is a feasible diagnostic tool in labor, and fetal lie, station, position, presentation, and attitude can be examined. This review presents an overview of fetal descent examined with ultrasound. Ultrasound was first introduced for examining fetal descent in 1977. The distance from the sacral tip to the fetal skull was measured with A-mode ultrasound, but more convenient transperineal methods have since been published. Of those, progression distance, angle of progression, and head-symphysis distance are examined in the sagittal plane, using the inferior part of the symphysis pubis as reference point. Head-perineum distance is measured in the frontal plane (transverse transperineal scan) as the shortest distance from perineum to the fetal skull, representing the remaining part of the birth canal for the fetus to pass. At high stations, the fetal head is directed downward, followed with a horizontal and then an upward direction when the fetus descends in the birth canal and deflexes the head. Head descent may be assessed transabdominally with ultrasound and measured as the suprapubic descent angle. Many observational studies have shown that fetal descent assessed with ultrasound can predict labor outcome before induction of labor, as an admission test, and during the first and second stage of labor. Labor progress can also be examined longitudinally. The International Society of Ultrasound in Obstetrics and Gynecology recommends using ultrasound in women with prolonged or arrested first or second stage of labor, when malpositions or malpresentations are suspected, and before an operative vaginal delivery. One single ultrasound parameter cannot tell for sure whether an instrumental delivery is going to be successful. Information about station and position is a prerequisite, but head direction, presentation, and attitude also should be considered.
Keywords: descent, engagement, head direction, labor outcome, station, ultrasound.
For labour to progress well, dilatation of the cervix should be accompanied by descent of the fetal head, which is plotted on the same section of the partograph, but using O as the symbol. But before you can do that, you must learn to estimate the progress of fetal descent by measuring the station of the fetal head, as shown in Figure 4.3. The station can only be determined by examination of the woman’s vagina with your gloved fingers, and by reference to the position of the presenting part of the fetal skull relative to the ischial spines in the mother’s pelvic brim.
Figure 4.3 Assessing the station (descent) of the fetal head by vaginal examination, relative to the ischial spines in the mother’s pelvic brim. (Source: WHO, 2008, Midwifery Education Module: Managing Prolonged and Obstructed Labour, Figure 7.28, page 132)
As you can see from Figure 4.3, when the fetal head is at the same level as the ischial spines, this is called station 0. If the head is higher up the birth canal than the ischial spines, the station is given a negative number. At station –4 or –3 the fetal head is still ‘floating’ and not yet engaged; at station –2 or –1 it is descending closer to the ischial spines.
If the fetal head is lower down the birth canal than the ischial spines, the station is given a positive number. At station +1 and even more at station +2, you will be able to see the presenting part of baby’s head bulging forward during labour contractions. At station +3 the baby’s head is crowning, i.e. visible at the vaginal opening even between contractions. The cervix should be fully dilated at this point.
Now that you have learned about the different stations of fetal descent, there is a complication about recording these positions on the partograph. In the section of the partograph where cervical dilatation and descent of head are recorded, the scale to the left has the values from 0 to 10. By tradition, the values 0 to 5 are used to record the level of fetal descent. Table 4.1 shows you how to convert the station of the fetal head (as shown in Figure 4.3) to the corresponding mark you place on the partograph by writing O. (Remember, you mark fetal descent with Os and cervical dilatation with Xs, so the two are not confused.)
When the baby’s head starts crowning (station +3), you may not have time to record the O mark on the partograph!
| –4 or –3 | 5 |
| –2 or –1 | 4 |
| 0 | 3 |
| +1 | 2 |
| +2 | 1 |
| +3 | 0 |
What does crowning mean and what does it tell you?
Crowning means that the presenting part of the baby’s head remains visible between contractions; this indicates that the cervix is fully dilated.
Page 2
Figure 4.4 Sutures and fontanels in the newborn’s skull.
The five separate bones of the fetal skull are joined together by sutures, which are quite flexible during the birth, and there are also two larger soft areas called fontanels (Figure 4.4). Movement in the sutures and fontanels allows the skull bones to overlap each other to some extent as the head is forced down the birth canal by the contractions of the uterus. The extent of overlapping of fetal skull bones is called moulding, and it can produce a pointed or flattened shape to the baby’s head when it is born (Figure 4.5).
Figure 4.5 Normal variations in moulding of the newborn skull, which usually disappears within 1–3 days after the birth.
Some baby’s skulls have a swelling called a caput in the area that was pressed against the cervix during labour and delivery (Figure 4.6); this is common even in a labour that is progressing normally. Whenever you detect moulding or caput formation in the fetal skull as the baby is moving down the birth canal, you have to be more careful in evaluating the mother for possible disproportion between her pelvic opening and the size of the baby’s head. Make sure that the pelvic opening is large enough for the baby to pass through. A small pelvis is common in women who were malnourished as children, and is a frequent cause of prolonged and obstructed labour.
Figure 4.6 A caput (swelling) of the fetal skull is normal if it develops centrally, but not if it is displaced to one side.
To identify moulding, first palpate the suture lines on the fetal head (look back at Figure 1.4 in the first study session of this Module) and appreciate whether the following conditions apply. The skull bones that are most likely to overlap are the parietal bones, which are joined by the sagittal suture, and have the anterior and posterior fontanels to the front and back.
- Sutures apposed: This is when adjacent skull bones are touching each other, but are not overlapping. This is called degree 1 moulding (+1).
- Sutures overlapped but reducible: This is when you feel that one skull bone is overlapping another, but when you gently push the overlapped bone it goes back easily. This is called degree 2 moulding (+2).
- Sutures overlapped and not reducible: This is when you feel that one skull bone is overlapping another, but when you try to push the overlapped bone, it does not go back. This is called degree 3 moulding (+3). If you find +3 moulding with poor progress of labour, this may indicate that the labour is at increased risk of becoming obstructed.
When you document the degree of moulding on the partograph, use a scale from 0 (no moulding) to +3, and write them in the row of boxes provided:
0 Bones are separated and the sutures can be felt easily.
+1 Bones are just touching each other.
+2 Bones are overlapping but can be separated easily with pressure by your finger.
+3 Bones are overlapping but cannot be separated easily with pressure by your finger.
In the partograph, there is no specific space to document caput formation. However, caput detection should be part of your assessment during each vaginal examination. Like moulding, you grade the degree of caput as 0, +1, +2 or +3. Because of its subjective nature, grading the caput as +1 or +3 simply indicates a ‘small’ and a ‘large’ caput respectively. You can document the degree of caput either on the back of the partograph, or on the mother’s health record (if you have it).
Imagine that you are assessing the degree of moulding of a fetal skull. What finding would make you refer the woman in labour most urgently, and why?
If you found +3 moulding and the labour was progressing poorly, it may mean there is uterine obstruction.
Page 3
You already know that good uterine contractions are necessary for good progress of labour (Study Session 2). Normally, contractions become more frequent and last longer as labour progresses. Contractions are recorded every 30 minutes on the partograph in their own section, which is below the hour/time rows. At the left hand side is written ‘Contractions per 10 mins’ and the scale is numbered from 1–5. Each square represents one contraction, so that if two contractions are felt in 10 minutes, you should shade two squares.
On each shaded square, you will also indicate the duration of each contraction by using the symbols shown in Figure 4.7.
Figure 4.7 Different shading on the squares you draw on the partograph indicates the strength and duration of contractions.