Young RC, Goloman G: Mechanotransduction in rat myometrium: coordination of contractions of electrically and chemically isolated tissues. Reprod Sci 18(1):64,2011 Zhang Y, Akins tvlL, Murari K, et al: A compact fiber-optic SHG scanning endomicroscope and its application to visualize cervical remodeling during pregnancy. Proc Nat! Acad Sci USA 109(32):12878, 2012 Ziecik AJ, Derecka-Reszka K, Rzucidlo SJ: Extragonadal gonadotropin receptors, their distribution and function. J Physiol Pharmacol 43(4 Suppl 1):33, 1992 Zingg HH, Rozen F, Chu K, et al: Oxytocin and oxytocin receptor gene expression in the uterus. Recent Prog Horm Res 50:255, 1995
MECHANISMS OF LABOR .e.........e..e.........e... 421
FETAL POSITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423
OCCIPUT ANTERIOR PRESENTATION ..e.......e..... 427
NORMAL LABOR CHARACTERISTICS . . . . . . . . . . . . . . . 431 ......................... 432
SECOND STAGE OF LABOR ..e...e.......e.....e..e..e. 434
MANAGEMENT OF NORMAL LABOR ...... ........ 434
MANAGEMENT OF FIRST-STAGE LABOR ...e.....e...e. 436
MANAGEMENT OF SECOND-STAGE LABOR ... ..e..e. 438
LABOR MANAGEMENT PROTOCOLS . . . . . .. . . . . . . . . 438
It olows that some process of adaptation or accommodation of suitable portions or the head to the various pelvic planes is necessary to insure the completion of childbirth.
his is brought about by certain movement of the presenting part, which belong to what is termed the mechanism of labour.
-J. Whitridge Williams (1903)
Labor is the process that leads to childbirth. It begins with the onset of regular uterine contractions and ends with delivery of the newborn and expulsion of the placenta. Pregnancy and birth are physiological processes, and thus, labor and delivery should be considered normal for most women.
Many adaptive changes are required for pregnancy and for labor and delivery. According to Nygaard (2015), vaginal delivery is a traumatic event. To assess this in part, Staer-] ensen and colleagues (2015) obtained transperineal sonographic measurements of the pelvic loor muscles at 21 weeks' and 37 weeks' gestation, and again at 6 weeks, 6 months, and 12 months postpartum. In 300 nulliparas, they measured bladder neck mobility and the area within the urogenital hiatus during Valsalva. This hiatus is the U-shaped opening in the pelvic loor muscles through which the urethra, vagina, and rectum pass (Chap. 2, p. 19). In this study, the levator hiatus area was signiicantly larger at 37 weeks' gestation and at 6 weeks postpartum compared with earlier pregnancy. hen, by 6 months postpartum, the hiatus had improved and narrowed to return to an area comparable to that at 21 weeks' gestation. However, no further improvement was noted by 12 months postpartum. Of note, hiatal area enlargement was only seen in those who delivered vaginally.