Presentation – the part of the fetus that directly overlies the lower pole of the uterus/pelvic inlet. A fetus in longitudinal lie is suitable for vaginal delivery. Several variables in the fetus influence its journey through the birth canal.įetal size can be estimated by palpation, ultrasound scan and customized growth chart but all of these methods are subjected to large degree of error.įetal lie is the relationship of the long axis of the fetus relative to longitudinal axis of the uterus. 5, 6 Fetus (passenger)įor a successful outcome, the fetal skull, shoulders, trunk and buttocks should pass through maternal pelvis. Uterine activity varies in different stages of labor: latent phase approximately 100 MVUs, active phase of labor 175 MVUs and 250 MVUs during the second stage. This is measured in Montevideo units (MVU). Quantitative assessment of intrauterine pressure to measure the strength of uterine contraction is done by placement of an intrauterine catheter. External tocodynamometry is a qualitative measurement of uterine activity, records uterine activity and correlates fetal heart rate (FHR) pattern with uterine contraction. The uterine contraction is characterized by its intensity, frequency, and duration. The pelvic inlet has a wide transverse diameter – approximately 13 cm, the midcavity of the pelvis is round, whilst the outlet has a wide anterior posterior diameter. It is divided into three parts – inlet, cavity and outlet (Figure 1). The true pelvis is shallow anteriorly, formed by the symphysis pubis (4–5 cm), and deep posteriorly, formed by the sacrum and coccyx (10 cm). The significance of the false pelvis is to support the pregnant uterus the true pelvis is a bony passage for fetus to pass during labor. The pelvic brim extends from the sacral promontory, along the ilium on each side circularly along the ridge divides the pelvis into upper false pelvis and lower true pelvis. The bones are articulated together by four joints: anteriorly symphysis pubis, two sacroiliac joints posteriorly and the sacrococcygeal joint inferiorly. The maternal pelvis is made of five bones (Figure 1): the sacrum and coccyx posteriorly, two innominate bones on each side, and the pubic bone anteriorly. Maternal pelvis (passage)īony pelvis: ilium, ischium, pubis, sacrum and coccyx. 2, 3, 4 MECHANISM OF NORMAL LABORįor a successful normal labor a coordinated interaction of the uterine activity (power), maternal pelvis (passage) and fetus (passenger) is required. The onset of labor is also associated with an increase in prostaglandin production in the placental and cervix, furthering inducing their receptors and facilitating cervical ripening (PGE 2) and uterine contractions (PGF 2a). 1 This potentiates oxytocin receptors in the myometrium, reduces the progesterone/estrogen ratio and upregulates myometrial gap junctions to facilitate uterine contractions. Fetal dehyroepiandrosterone sulfate (DHEAS) is converted to estriol and estradiol by the placenta. Complex interactions of hormones between uterus, placenta and fetus. The factors that trigger labor at term are not clearly understood it is postulated that it is a result of changes in the hypothalamic–pituitary–adrenal axis, increasing fetal cortisol, and placental enzymatic functions. World Health Organization defines normal labor as starting spontaneously at term (37 completed weeks of gestation) for a fetus with cephalic presentation, progressing without maternal or fetal complication, and resulting in the delivery of fetus followed by placenta and membranes. The mechanism of normal labor is series of events that take place in the genital organ that allow the birth of a viable fetus at term followed by expulsion of placenta and membrane from the vagina. See end of chapter for details INTRODUCTION By completing 4 multiple-choice questions (randomly selected) after studying this chapter readers can qualify for Continuing Professional Development awards from FIGO plus a Study Completion Certificate from GLOWM
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