In this article we’ll be highlighting the normal mechanisms of a standard, low-risk labor and delivery. 

What are the signs an individual is in labor?

Labor usually begins between the 38th and 42nd week of gestation. Some early signs include the baby dropping down into the pelvic inlet (this is referred to as engagement). When this happens, the uterus moves downward a bit and the fundus doesn’t press into the diaphragm as much and the parent can breathe more easily (they are going to need that respiratory capacity in order to get through labor!). This is also the time when the individual will experience sporadic Braxton-Hicks contractions. The cervix ripens (becomes more soft) and the blood-tinged mucus plug is lost…typically around 24-48 hours prior to official labor onset. Some other signs can be a sudden burst of energy in the day or two prior to labor, increased backache, GI upset and sacroiliac pressure. 

How do you differentiate between true and false labor?

DILATION Progressive dilation No progression in dilation
EFFACEMENT Progressive effacement No progression in effacement
PAIN RELIEF Not relieved by ambulation or rest Relieved by ambulating, changing position, resting, hot bath/shower
CONTRACTIONS Occur regularly and increase in frequency, duration and intensity Irregular
PAIN DESCRIPTION Usually starts in back and radiates to abdomen Perceived as hardening, mainly felt in lower abdomen and groin

What happens to the patient’s physiology during labor?

  • When the uterus contracts, the blood within the organ is returned to systemic circulation, which increases preload and, therefore, cardiac output. Cardiac output also increases during labor due to increases in heart rate and stroke volume. 
  • Blood pressure will change depending on the laboring individual’s position or the stage of labor
    • First Stage: increases by 35/25 mm Hg
    • Second Stage: increase diastolic by 65 mm Hg
    • Position changes due to compression on vena cava, which drops BP.
  • Tachypnea is common as the body works to meet increased oxygen demands. This can lead to respiratory alkalosis (a decrease in carbon dioxide). Evidence shows that opioids help somewhat, but epidural anesthesia is the most effective for preventing respiratory alkalosis. 
  • Fluid and electrolyte imbalances can occur due to excessive sweating and hyperventilation. Hyperventilation can lead to respiratory alkalosis, which can then lead to hypocalcemia and hypokalemia. 
  • Urine output increases as the cardiac output increases. You’ll see increased GFR on your renal panel.
  • Gastric emptying is slower during labor, and this is exacerbated by opioid administration. This can lead to the patient having higher gastric volumes during this time (which can be troublesome if emergent surgery or intubation is needed). 
  • White blood cell counts can go up to 25-30K
  • Blood glucose decreases because it is being used as a source of energy

What happens to baby’s physiology during labor?

  • The fetal heart rate will change due to stress of labor. Additionally, inadequate blood flow and the patient’s position can affect the heart rate. We’ll talk about fetal heart rate monitoring in another post.
  • If the fetus is under stress (such as with low oxygen levels or infection), he’ll take in forceful breaths in utero and aspirate meconium. 

What are the 6 P’s of labor?

P #1: Passenger

This refers to the size and position of the fetus (also the number of passengers!)

  • Skull diameter in relation to position: We look at the pelvic inlet size and compare that to the diameter of the baby’s head. It’s important to note that if baby is in the normal position, the fetal diameter is much smaller than if he’s in the face-up position. You can see an example of this here.
  • The baby’s skull bones can overlap a bit (0.5 to 1cm), which allows for baby’s head to be molded to the pelvis size for delivery. You can typically feel this occurring toward the back of the head and may feel a ridge there.
  • Fetal attitude refers to the relation of his body parts to one another. The normal position is “general flexion.” The baby’s head is flexed so his chin is on his chest with his legs flexed at the knees and thighs. Changes in fetal attitude cause the baby to present with larger diameters of his head to the pelvis.
  • The most common presentation is the vertex or flexed presentation, where the occiput is the presenting part and the fetal head is completely flexed onto his chest. This puts the smallest diameter of the head presenting to the pelvis (9.5 cm)
  • Fetal lie refers to the relationship of the long axis (spine) of the fetus to the long axis of the parent (transverse and longitudinal). A longitudinal lie is present with an uncomplicated labor.
  • Station refers to the relationship of the presenting part to an imaginary line drawn between the ischial spines of the pelvis. In a normal pelvis, the space between the ischial spines is the narrowest diameter and is designated “zero station.”
  • Fetal position refers to the relationship of the landmark on the presenting fetal part to the anterior, posterior or sides of the pelvis. Recall that the occiput is the landmark in the vertex or “flexed” position

P # 2: Passageway

The main concept here is that the bony boundaries of the pelvis are absolute. The baby is either going to fit through the pelvis or not. The passageway is comprised of the bony pelvis and the soft tissue  (lower uterine segment, cervix, vagina and perineum). There are four different types of bony pelvis: 

  • Gynecoid (most common, about 50% are this type). The general shape is round and shallow and is the most favored shape for an uncomplicated delivery. 
  • Anthropoid (~24%). This pelvis is narrow and deep…think of an egg shape. Labor may be longer than with a gynecoid pelvis.
  • Android (~23%). This pelvis shape is more like a male pelvis. It is more narrow than the gynecoid shape. Labor may be longer and more difficult, so individuals with this shape may need a C-section.
  • Platypelloid (~3%). This pelvis shape is wide and shallow…think of an egg lying sideways. Note that with this pelvis type the baby is often delivered face up, and many individual’s with this pelvis shape will have a C-section.

The pelvic floor muscles draw the rectum and vagina upward and forward with every contraction. Additionally, the perineal thickness decreases making these tissues more pliable. However, they can tear during childbirth with the severity of the tear based on how much it extended into the lining of the anus or rectum. 

P #3: Power (aka: the physiological forces of labor)

The primary force of labor are the involuntary uterine contractions with that upper portion being the contractile segment. Secondary forces are present when the patient uses abdominal muscles and bears down to push out the fetus. 

Effacement is the drawing up of the internal os and cervical canal into the uterine side walls. Essentially the cervix changes from a long, thick structure to a stretched, thin structure. 

Dilation simply means that the cervix itself is open so the fetus can pass through. In primagravidas (first timers) effacement will typically precede dilation. When the cervix is completely dilated, it’s at about 10cm.

P #4: Position (of the laboring individual)

The most commonly used position for labor and delivery is the lithotomy position, namely due to continuous fetal monitoring and the use of opioids or epidurals. However there are other positions utilized, and these include

  • Upright positions include standing/squatting, kneeling, sitting on a birthing seat or hands-and knees. Benefits include gravity and less risk of compressing aorta. It is also thought that upright positions enable the uterus to contract more strongly and facilitate optimal fetal position. 
  • Recumbant/semi-recumbant positions include supine and lithotomy
  • Lateral position has the individual lying on their side

P #5: Placenta

In a normal, uncomplicated birth, the placenta detaches from the uterine wall and is expelled within a few minutes up to one hour after delivery. 

P #6: Psyche

It’s also important to understand the role of the psychology of birth, especially as it relates to the parent. This includes their anxiety or fear about the birthing process, their knowledge of labor and delivery, trust in the medical team, as well as their beliefs and culture. It’s also affected, of course, by the parent’s support person, whether or not a doula is utilized, and the health care providers’ practices.

What are the stages of labor? 

  • First stage of labor involves effacement and dilation. It typically lasts about 12 hours for a first baby, but is quicker for subsequent births (about 7-8 hours). This first stage has three sub-stages:
    • Latent phase or “early labor.”  This stage begins at the onset of regular contractions which increase in frequency, duration and intensity. Mild contractions last 30 to 45 seconds and occur approximately every 5 to 30 minutes. The individual is able to rest or sleep during this time and the cervix opens to about 3-4 cm. For primigravidas, this is about 6 hours. For multigravida, it’s typically about 4 hours. With all contractions, you are assessing for regularity or irregularity, frequency, duration and intensity.
      As you palpate the fundus during a contraction, a mild contraction will feel like you’re pressing on a cheek, a moderate contraction will feel about as firm as the end of your nose and an intense or strong contraction will feel as firm as your forehead.
    • Active phase: The patient’s anxiety increases in this stage as the contractions intensify in duration and come more frequently, typically every 3-5 minutes and lasting approximately 60 seconds in duration. The cervix dilates from 4-7 cm as fetal descent progresses. During this time, the laboring individual will be more anxious and restless. Primagravidas can expect to be in this phase for about 3 hours, while multigravidas typically get through this phase in about 2 hours.
    • Transition phase: There is significant anxiety in this phase as contractions become more frequent, longer and much more intense (occurring every 2-3 minutes and lasting 45-90 seconds). The cervix opens to 10cm during this time. The individual may be nauseous and have difficulty concentrating as the contractions become increasingly intense. Agitation and even fear during this time are common. The patient will have an urge to push and feel increased rectal pressure. This is the most difficult part of labor and typically lasts about 20 to 40 minutes.
  • Second stage of labor. This stage begins with full dilation and continues until the baby is born. In a first-time laboring individual, this can take 30-120 minutes, while multigravidas will have a shorter stage of around 5-30 minutes. The patient is actively pushing during this time.
  • Third stage of labor involves delivery of the placenta. This generally takes 5-30 minutes. If the shiny side of the placenta emerges first, this is referred to as “Schultze presentation.” If the more dull side emerges first, this is called “Duncan presentation.” 
  • Fourth stage of labor is essentially the recovery period and lasts from the moment of delivery through the time the parent’s vital signs are stabilized. The patient is at risk for hemorrhage during this time, so you’ll monitor closely for about 2 hours. It’s also when we do skin-to-skin with baby for that all-important bonding. If the patient isn’t available or able, then a significant other can definitely step in for some one-on-one time with the little one. 

I hope this helps you understand what to expect in a low-risk, uncomplicated labor and delivery. To learn more about complex L&D, check out this post on preeclampsia, this one on placenta previa vs placental abruption and this one on adolescent pregnancy.


Study this topic on the go in episode 146 of the Straight A Nursing podcast! You can stream it from here or wherever you get your podcast fix.


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