Amniotomy, also known as artificial rupture of membranes (AROM) and by the lay description "breaking the water," is the intentional rupture of the amniotic sac by an obstetrical provider. This procedure is common during labor management and has been performed by obstetrical providers for at least a few hundred years. The reasons for intentional rupture of the amniotic sac during labor are multifold and include, but are not limited to, influencing the speed of labor, allowing for more direct monitoring of fetal status, and qualitative assessment of the amniotic fluid.
The amniotic cavity is an enclosed space within the uterus in which the fetus develops and is protected during the antepartum period. The amniotic cavity is comprised of a dual layer membrane, which includes an inner layer known as the amnion and an outer layer known as the chorion. This potential space forms early in pregnancy and is filled with serous fluid during the first few weeks of pregnancy. With further development of the fetus, specifically the fetal urinary system, the fluid in this potential space increases as the developing fetus excretes urine. Fetal urine comprises most of the amniotic fluid. Typically, this barrier remains intact throughout the duration of gestation, and the amniotic membranes will spontaneously rupture, releasing the amniotic fluid either immediately preceding spontaneous labor or sometime after the onset of spontaneous labor.
The two principal reasons for artificial rupture of membranes are (1) to induce or augment the labor process or (2) to assist in placement of internal fetal monitoring to provide the direct assessment of fetal status. Monitoring of the fetal heart rate as well as uterine activity can be easily obtained via external monitoring systems. However, in certain circumstances, more direct evaluation of the fetal heart rate or uterine activity is required during labor. The amniotic membrane presents a physical impediment to this form of monitoring, and to place a fetal scalp electrode or intrauterine pressure catheter the membranes must necessarily be broken before placement.
Contraindications to this procedure are few and obvious. Artificial rupture of membranes should not be undertaken in the case of malpresentation, vasa previa, or an unengaged fetal head.
Amniotomy is easily performed with the use of specially designed hooks intended to grab and tear the amniotic membrane. The two most commonly used devices are (1) an approximately 10-inch rod with a hook on the end of the rod or (2) a finger cot with a hook on the end of the cot. With either device, the practitioner first assesses cervical dilation through the performance of a sterile digital exam. At the same time, assessment of the fetal presenting part is made, ensuring that the presenting part is, in fact, the fetal head and assessing that the fetal head is well engaged in the pelvis. After confirmation of both fetal presentation and engagement, the practitioner can proceed with artificial rupture of membranes.
As an example, when using the rod-hook device, the end of the rod that remains outside of the vagina is typically held with the nondominant hand. The hook end of the rod is then protected between two fingers when entering the vagina. When the practitioner can palpate the amniotic membrane and the presenting part, the nondominant hand is used to advance the hook to the amniotic membrane. The membrane is then snagged with the hook, and gentle traction is applied in a superior direction to tear the amniotic membrane. Successful rupture of membranes most commonly is determined by the immediate return of amniotic fluid from the vagina. This fluid usually is clear and odorless. However, in certain circumstances, the fluid may either contain meconium or may be blood tinged. It is important to note the color of the fluid at the time of rupture. Typically, following artificial rupture of membranes, the practitioner should not immediately remove their hand from the vagina because it is at this point that the highest risk of potential cord prolapse can occur and will be noted as the amniotic fluid continues to drain. After the immediate flow of amniotic fluid ceases, and there is no palpable cord in the vagina, the vagina hand then can be removed.
Complications also are relatively few. Rupture of membranes does eliminate the primary barrier between the fetus and the polymicrobial environment of the vagina. If performed too early in the labor process, there can be an increased risk of intrapartum chorioamnionitis. The most common complication of artificial rupture of membranes is prolapse of the umbilical cord. This invariably occurs after rupture if artificial rupture of membranes is performed when the head is not engaged in the maternal pelvis. In the case of an unengaged fetal head, rupture of membranes may allow for the umbilical cord to precede the fetal head when the release of amniotic fluid occurs. This will allow the fetal head to compress the section of umbilical cord preceding the head, generally leading to fetal bradycardia and necessitating emergency cesarean section. This complication should be an easily avoidable, iatrogenic cause of emergency delivery.
Practitioners have believed that artificial rupture of membranes either can assist in inducing labor or augmenting spontaneous labor. It is commonly felt that relieving the amniotic sac of amniotic fluid induces uterine contraction activity, increases the strength of contractions, and may augment labor by allowing direct pressure from the fetal scalp on the uterine cervix which may assist in dilating the cervix. While these are commonly held beliefs of many practitioners, the data to support amniotomy for these reasons is uncertain. Some studies have produced data supporting the practice, while others suggest that this practice does not, in fact, accomplish any of these outcomes. As a result, meta-analysis has been performed; however, the data has been mixed. Meta-analysis seems to suggest that amniotomy to shorten spontaneous labor does not produce a discernible difference when compared to no amniotomy. Nevertheless, when there is a delay in labor, the meta-analysis suggests there is a modest reduction in the rate of cesarean section when rupture of membranes in conjunction with the use of oxytocin is used as an early intervention.