A few weeks prior to your due date, you'll start experiencing Braxton-Hicks contractions. These can be identified by watching your HUD (the pink heart will blur slightly and pulse for the duration of the contraction). Additionally, your avatar will emote a brief scowl, and the HUD may say "You're experiencing a mild contraction." Braxton-Hicks contractions can begin as early as the 27th week, or as late as the 34th week (assuming a 40-week gestation). They will increase in frequency from about every once per RL day to about every 12 RL minutes as your due date arrives. B-H contractions last about five seconds.
When labor starts, your HUD will indicate your labor stage, and how far you've progressed through it. Labor progression is not affected by any acceleration or day length configured in your HUD, so all times stated below are real life (RL) times. However, the different stages of labor progress at different rates each pregnancy, hence the ranges indicated.
When in labor, you can click your HUD to control its progression (coupling, configuration, and other options are not available during labor). At any point, you may choose the "Restart" option to return to the beginning of active labor. Each stage also offers the option to progress to the next stage immediately.
Active Labor
Active labor is the first stage, and takes about 20-40 RL minutes to complete. During active labor, the cervix dilates from 3cm to 7cm. Moderate contractions initially arrive every 4 RL minutes and last about 30 seconds. As active labor progresses, contractions increase in frequency to about evey two RL minutes, and last up to 90 RL seconds.
You may skip over this stage of labor by clicking your HUD and choosing "Transition".
Active Labor - Transition
The transition stage is the most intense portion of labor, and lasts 10 to 20 RL minutes. The cervix dilates rapidly from 8cm to 10cm. Strong contractions arrive regularly every two minutes and last 60 to 90 seconds.
You may skip over this stage of labor by clicking your HUD and choosing "Delivery".
Delivery
Delivery is the final stage, and begins when the cervix is fully dilated (10cm). It lasts from 10-20 RL minutes, but can be shortened by pushing during contractions. Contractions arrive every 30-60 RL seconds and last about 20 seconds. When a contraction is occurring, you can click your HUD and select "Push"; if you do so, the length of this stage will shorten. You must select "Push" while the contraction is active; otherwise, it will have no effect.
When you reach 100%, you can select "Deliver!", and the gender and father of your baby will be revealed. If you were carrying only one fetus, you'll be returned to the first day of your cycle within a few moments. Otherwise, the Delivery stage will repeat, likely with a slightly different duration.
At any point, you may skip forward and select "Deliver!"