The delivery of a full-term newborn refers to delivery at a gestational age of 37-42 weeks, as determined by the last menstrual period or via ultrasonographic dating and evaluation. The Naegel rule is a commonly used formula to predict the due date based on the date of the last menstrual period. This rule assumes a menstrual cycle of 28 days and mid-cycle ovulation. Ultrasonographic dating can be more accurate, especially when it is performed early in pregnancy and is used to corroborate or modify a due date based on the last menstrual period.
Approximately 11% of singleton pregnancies are delivered preterm and 10% of all deliveries are postterm. Thus, nearly 80% of newborns are delivered at full term, although only 3-5% of deliveries occur on the estimated due date. Over the past few decades, the number of patients who go into spontaneous labor has decreased, and the percentage of inductions (iatrogenic labor) has increased to 22% of all pregnancies.
Labor and delivery is divided into 3 stages.
- In the first stage, the cervix dilates as a result of progressive rhythmic uterine contractions. This is typically the longest stage of labor. Cervical effacement, or thinning, occurs throughout the first stage of labor, and is graded 0-100%.
- The first stage of labor is divided into the latent and active phases.
- The latent phase can last for many hours. The cervix dilates, usually slowly, from closed to approximately 4-5 cm.
- The active phase lasts from the end of the latent phase until delivery. It is characterized by rapid cervical dilation. The cervix usually dilates at a rate of 1.0 cm/h in nulliparous women and 1.2 cm/h in multiparous women during the active phase.
- The second stage of labor is the time between complete cervical dilation and delivery of the neonate. This phase lasts minutes to hours. The maximum accepted time for the second stage depends on the patient's parity and whether the patient has an epidural.
- Six cardinal movements of labor occur during the second stage of labor.
- Engagement of the head into the lower pelvis
- Flexion of the head, putting the occiput in presenting position
- Descent of the neonate through the pelvis
- Internal rotation of the vertex to maneuver past the lateral ischial spines
- Extension of the head to pass beneath the maternal symphysis
- External rotation of the head after delivery to facilitate shoulder delivery
- Several clinical parameters are followed.
- The fetal presentation is determined by the first fetal body part that passes through the birth canal. Most commonly, this is the occiput or the vertex of the head.
- The fetal station is the relation of the fetal head to the maternal ischial spines. The station is defined as -5 cm to +5 cm; 0 station is at the level of the ischial spines.
- The fetal position is the orientation of the fetal vertex (the top of the head) in relation to the plane of the maternal ischial spines. The vertex normally rotates from a transverse position to an anterior or posterior position as the vertex internally rotates.
- The delivery of the placenta is the third and final stage of labor; it normally occurs within 30 minutes of delivery of the newborn. As the uterus contracts, a plane of separation develops at the placenta-endometrium interface. As the uterus further contracts, the placenta is expelled.
In November 2013, the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) released a committee opinion revising the definition of term pregnancy. The recommended change, as devised by a work group that included representatives from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, ACOG, SMFM, and other societies and organizations, replaces the designation “term” with the following :
- Early term: 37 weeks, 0 days, through 38 weeks, 6 days, of gestation
- Full term: 39 weeks, 0 days, through 40 weeks, 6 days, of gestation
- Late term: 41 weeks, 0 days, through 41 weeks, 6 days, of gestation
- Postterm: At least 42 weeks, 0 days, of gestation
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