Normal Labor

XIE MEIQING

29/09/2006

Labor means the process of the birth,which is finished by the effective coordination of uterine contractions and pushing-down efforts, the lower segment stretching, effacement and dilatation of cervix. It is a process. Labor begins from the onset of uterine contraction and terminate with the expulsion of the products of pregnancy.

There is another term—

delivery,which describes the

phenominum of the actual expulsion

of the products: fetus and

placenta.But it has the same

meaning as labor in Chinese.

 Several definitionsabortion:< 28 week pregnancylabor:≥28 week pregnancypreterm labor :28 → < 37 week pregnancyterm labor:≥37→ < 42 week pregnancypostterm labor :≥ 42 week pregnancy

Labor can be normal or abnormal.It is influenced by three essential factors ， We can named the three factors in “ The Three P”

the three P1. the powers ---- forces2. the passage ---- birth canal3. the passenger ---- fetus(baby) and placenta

The power1 uterine contraction : is

involuntary, rhythm, symmetry and polarity, called retraction  

2 abdominal wall contraction and lelator ani contraction :called bearing—down efforts,is voluntary ， which help to deliver the baby after the cervix os fully dilated.They create from the contraction of abdominal wall muscle and diaphrum muscle and lebator ani muscle.

a.      rhythm: the uterine contractions are rhythmical\ periodic and progressive.which called “Pains”. In the very beginning of labor, each contraction last about 30 seconds, and the interval between two contractions is 5 to 10 minutes, as the labor progresses, each contraction may last longer and longer, and the intervals may become shorter and shorter and be stronger and stronger. And when the cervix fully dilates,the uterine contraction may be as longer as 60 seconds,and the intervals may be 1 to 2 minutes.

b.      Symmetry and polarity: the contractions create from two poles of uterus, then go to the fundus and upper part of the uterus, then go down to the lower segment , the forces of the upper segment are more powerful than that of lower segment of uterus.c.      Retraction: after each contraction the length of each myometrium muscle of the uterus can not return to the former length, it becomes shorter and shorter. The uterine capacity is progressively reduced so the pressure inside uterus becomes stronger and stronger.

  The passage

1 Bony passage 2 Soft passage

Bony passage : also called the true pelvis , it composed of four bones: sacrum, coccyx and two innominate bones, each innominate bone is formed by the fusion of the ilium, ischium and pubis.

Because the shape and size of the true pelvis are crucial in labor so each diameter of the pelvis is very important.If each diameter fits the baby the labor will be finished easily, if not abnormal labor will occur. For easy understanding, We can imagine four planes of the true pelvis: the inlet(brim) widest plane mid plane outlet

a.      The plane of the inlet( the brim): the brim is bounded anteriorly by the pubis, laterally by the iliopectineal lines,posteriorly by the sacrum promontory, four diameters are mentioned here:

the antero- posterior diameter------from the superior border of the pubic symphysis to the middle of the sacral promontory, measures 11cm..the transverse diameter------- between two illiopectineal,measures 13cm.The oblique diameters------ two oblique diameters, left oblique diameters from left sacro-iliac joint to the right iliopubic tubercle. Right one from right sacrro-iliac joint to the left iliopubic tubercle, mesrers 12.75cm.

b.      The mid plane of pelvis:this is the smallest and most important plane.two diameters will be mentioned here:

The antero-posterior diameter------from the apex of the pubic arch through the middle of the transverse diameter of ischium spines to the sacrum,measures 11.5cm..The transverse diameter------ between two ischial spines meassures 10cm. This is the most important diameter when baby head descends through the mid plane of the pelvis. abnormal labour may occur when it is shorter than normal.

c.      The plane of outlet: the lowerest boundary of the bony passage and abnormal labor may after occur here. Four diameters mentioned:

The anter-posterior diameter------from the inferior border of symphysis to the sacro-coccygeal joint,measures 11.5cm.The transvwese diameter------also called the inter- tuberous diameters,measures 9cm,which between the two ischial tuberosities, is the important diameter when the baby’s head going through the outlet of the pelvis.The anterior sagittal diameter------from the arch of the symphysis to the middle of the transverse diameter,measures 6cm.The posterior sagittal diameter------ from the sacro-coccygeal joint to the middle of the transverse diameter measures 8.5cm.

Soft passage :lower segment cervix vagina pelvic floor

1.    Formation of the lower segment of the uterus: it comes from the isthmus uteri, which is 1cm in non-pregnant uterus, it can be stretches much longer and become thinner during pregnancy and labor due to uterine contractions.

2. Changes of the cervix

(1)Effacement : The shortening of the cervical

canal is called effacement of the cervix; Before

labour, the length of the canal is about 2cm but

gradually shortened during labour and acts as a

functional part of the lower uterine segment.

(2) Dilatation of the cervical os : The cervical os

dilates gradually during labour. When the dilatation

has reached 10 cm in diameter, it is said "complete"

or "full". This is achieved by the contraction of the

uterus and the ligaments ,and also by the hydrostatic

action of the forewater bag.

3. Changes of the pelvic floor,vagina and

perineum

The stretching of the fibres of the levator ani and

the thinning of the central portion of the perineum

transforms to almost transparent membranous

structure.

The Passenger(the foetus)1. Foetal size

Large foetus (macrosomia) (weight>or=4kg)may

cause dystocia due to cephalo-pelvic disproportion.

a.   Biparietal diameter---BPD,most important, mesures 9.3cm in mature infant.b.Suboccipitobregmatic diameter--measures 9.5cm, fit the mid pelvis when the baby head going through c.Occipito frontal diameter--mesures 11.3cm, the engagement diameter.d.Occipitomental diameter—mesures 13.3cm, the largest diameter of the head.

2.Foetal positionAbnormal position or presentation may cause dystocia.

3. Foetal malformationMalformation of the foetus such as hydrocephalus , monster, conjoined twin may cause dystocia.