STAGES OF LABOR

Labor refers to the complex process through which uterine contractions bring
about progressive dilation/opening and effacement/thinning of the cervix
leading to descent of the fetus through the birth canal ending with expulsion of
the neonate from the mother’s body.

A labor curve shows the change in cervical dilation over time. Older studies
(Friedman, 1954) were based on 500 women at a single U.S. hospital. That labor
curve is not applicable to today’s obstetric patients. Today’s population has a
higher BMI than 60 years ago. This, along and changing obstetric and anesthesia
practices, have led to new normal labor curves based on more current data.

Newer studies (Zhang et al, 2010) based on >60,000 laboring women at 19 U.S.
medical centers produce contemporary labor curves and norms which differ
significantly from the older Friedman data. The new data suggest the following:

Labor   Stage Definition Function Duration

Stage   1—Latent

phase   Effacement

Begins: onset   of  regular uterine contractions

Ends:   acceleration    of  cervical    dilation

Prepares

cervix  for

dilation

<20 hours   in

primipara

<14 hours   in

multipara

Stage   1—Active Begins:    acceleration    of  cervical    dilation    Ends: Rapid ≥0.7

Transition  from    latent  to  active  phase   is  at  6   cm, rather  than    4   cm.

Rate    of  active  phase   cervical    dilation    curve   is  much    slower  than    previously

thought.