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What causes contractions to start?

Contractions are tightenings of the uterus that occur during pregnancy. They are perfectly normal and help the uterus prepare for labor and delivery. But what exactly causes contractions to start in the first place?

The Uterus and Contractions

During pregnancy, the uterus grows dramatically in size to accommodate the growing baby. The uterus is a large, muscular organ made up of smooth muscle tissue. Throughout pregnancy, mild contractions of the uterine muscles help blood flow properly to the fetus. These are known as Braxton Hicks contractions.

As a woman nears the end of her pregnancy, the uterus begins preparations for the intense work it will need to do during labor. The timing varies, but this prep work usually begins several weeks before the woman’s due date.

What Triggers Labor?

While we know the general timeline of preparation, scientists are still working to understand exactly what causes labor to begin. The complex hormonal changes that support pregnancy also likely help initiate labor. But the specific trigger that tells the uterus to switch from preparations to the active work of labor remains unknown.

Researchers have identified several factors that may contribute to starting contractions and inducing labor:

  • Changes in hormones – Estrogen and progesterone levels drop while oxytocin levels rise
  • Fetal development – The fetus secretes hormones that help trigger labor
  • Detachment of the placenta – Reduced blood flow signals it’s time for the baby to be born
  • Inflammation of the uterus – Contractions increase to fight infection
  • Stretching of the uterus – Reaching maximum capacity triggers contractions

While each of these factors plays a role, experts have not identified a definite “start button” that kicks off active labor. The timing seems to involve both fetal and maternal factors.

Stages of Labor

Labor itself occurs in three distinct stages:

  1. Early labor – The long, slow period of increasing contractions that efface and dilate the cervix. Contractions are generally mild and last about 30-45 seconds.
  2. Active labor – Contractions become stronger, longer, and closer together. This is the most intense period when the cervix dilates fully to 10 cm.
  3. Transition – The most difficult and shortest phase. Contractions are very frequent and intense as the body prepares to push.

While the early preparations for labor can begin weeks before delivery, active labor itself usually lasts 6-12 hours in first-time mothers. Subsequent labors are often much shorter.

Contractions and Cervical Dilation

Contractions serve an important mechanical purpose – to open (dilate) the cervix. The cervix is the lower part of the uterus that connects to the vagina. For most of pregnancy it is long, closed, and fairly rigid. For a vaginal delivery, this tight cervical passage must stretch and thin out (called effacement) so the baby can pass through.

Contractions gradually stretch open the cervix. Early, mild contractions begin this process. As labor progresses, the contractions become longer, stronger, and closer together. This drives the rapid dilation needed for the pushing stage. The chart below shows average cervical dilation during labor:

Stage Approx. dilation
Early labor 0-6 cm
Active labor 4-7 cm
Transition 8-10 cm

During the transition phase, the cervix typically dilates the final 2-3 cm needed for the baby to emerge. Contractions during this stage are the strongest.

What Do Contractions Feel Like?

The feeling of contractions can vary tremendously depending on the stage of labor. Here are some ways women often describe the sensation:

  • Menstrual cramps – Contractions feel like strong menstrual or intestinal cramps that come and go.
  • Tightening – The uterus feels like it is tightening or balling up.
  • Pain in abdomen, groin, back – Contracting muscles can radiate pain into the abdomen, sides, lower back, and thighs.
  • Pelvic pressure – Late labor contractions feel like intense downward pressure, often described as the baby “dropping.”
  • Takes your breath away – The strength of the contractions can make it difficult to talk or catch your breath.

Back labor is when contractions cause intense lower back pain. This happens when the baby is positioned so that he or she presses against the mother’s spine during contractions.

Monitoring Contractions

During labor, contractions are monitored to assess how the labor is progressing. Important factors include:

  • Frequency – How often contractions occur, measured from the start of one to the start of the next.
  • Duration – How long each contraction lasts.
  • Intensity – How strong the contractions feel to the mother.
  • Resting tone – How relaxed the uterus feels between contractions.

Patterns in these factors give clues about how labor is progressing. For example, contractions that get progressively closer, longer, and stronger usually indicate productive labor progress.

Contractions may be monitored by:

  • Timing – The mother times them herself with a watch.
  • Touch – A provider feels the uterus during a contraction.
  • External fetal monitoring – A belt around the abdomen tracks frequency.
  • Internal fetal monitoring – A sensor inserted through the cervix directly measures contractions.

Inducing Labor

While labor usually starts naturally between 37-42 weeks, there are certain situations where labor may need to be induced or artificially started:

  • The pregnancy has extended past the due date, increasing risks to mother and baby.
  • Labor stalls and contractions have stopped.
  • Problems with the placenta, fluid levels, or growth mean the baby should be born early.
  • The mother has health conditions like diabetes or high blood pressure.
  • The mother’s water breaks before labor begins.

Common medical methods for inducing labor include:

  • Oxytocin (Pitocin) – This labor hormone given through an IV makes contractions stronger and more regular.
  • Artificial rupture of membranes (AROM) – Breaking the amniotic sac can help trigger oxytocin release.
  • Cervical ripening agents – Medications like misoprostol soften and dilate the cervix.
  • Foley catheter – Placing a small balloon catheter in the cervix helps efface and dilate it.

There are also several natural methods women attempt to stimulate labor. Common ones include:

  • Nipple stimulation – Can increase oxytocin release
  • Sexual intercourse – Semen contains compounds to “ripen” the cervix
  • Walking and physical activity – May help position the baby
  • Castor oil – Laxative effect may stimulate the bowels and uterus
  • Acupuncture – Used to promote contractions

However, the effectiveness of these natural induction methods varies. Many women find that they do not reliably jumpstart labor.

Risks of Inducing Labor

While there are clear medical benefits to inducing labor when medically warranted, there are some risks as well. Potential risks of inducing labor include:

  • Failure to induce – The induction does not successfully trigger labor.
  • Longer labor – Induced labors may take longer.
  • Uterine rupture – Contractions may cause a rarely occurring uterine tear.
  • Distressed baby – The artificially forced contractions can distress the fetus.
  • Cord prolapse – The umbilical cord slips into the cervix before the baby, cutting off circulation.
  • Need for C-section – Induced labor has a higher risk of ultimately needing a cesarean delivery.

However, when medically indicated, the benefits of induction usually outweigh these risks. Careful monitoring during an induction can help reduce potential complications.

When to Go to The Hospital

It can be tricky to know when to go to the hospital or birthing center in early labor. Signs that active labor is establishing and you should seek medical care include:

  • Contractions 5 minutes apart or less for 1-2 hours.
  • Contractions are strong and regular.
  • Bloody show or ruptured membranes.
  • Can’t talk through contractions.
  • Vaginal bleeding.
  • Decreased fetal movement.

First labors tend to take much longer than subsequent labors. It’s ideal to labor at home as long as possible before going in. This helps avoid interventions like epidurals or C-sections due to failure to progress.

How to Time Contractions

Timing contractions yourself before heading to the hospital can provide helpful information to share with your medical team. Follow these tips for accurately timing contractions at home:

  1. Start timing when you first notice a contraction begins, not when it peaks.
  2. Stop timing when the contraction fully releases.
  3. Record the length from start to finish and how far apart one starts from the next.
  4. Time several in a row to identify a pattern.
  5. Note if you have to breathe through them or stop activity.
  6. Use an app, stopwatch, or clock to precisely time them.

Report details like average length, frequency, and intensity to your provider when you get to the hospital. Consistently increasing duration, frequency, and strength usually means you are in active labor.

Braxton Hicks vs Real Contractions

Throughout pregnancy, the uterus periodically contracts and tightens. Known as Braxton Hicks contractions or false labor, these help prepare the uterus for labor. Some key differences between Braxton Hicks and true labor contractions include:

Braxton Hicks Contractions Real Labor Contractions
Irregular Regular and consistent
Do not increase over time Increase in strength, duration, and frequency
Feels like tightening Feels like menstrual cramps or pain
May be felt only in front Usually wraps from back to front
Not particularly painful Progressively more painful
Go away with position changes Continue despite moving positions

While Braxton Hicks are a good sign the uterus is preparing for labor, they do not indicate active labor is starting or require medical attention. However, if you are ever uncertain if contractions are the real deal, contact your provider.

Contraction Patterns to Watch For

Certain contraction patterns can be an early warning sign of problems. Contact your provider right away if you notice any of the following:

  • Contractions every 1-3 minutes lasting over 60 seconds when labor is not yet established.
  • Extremely long contractions pushing 2-3 minutes.
  • More than 5 contractions per hour before 37 weeks.
  • Contracting without any resting tone/release in between.
  • Decreased fetal movement along with consistent contractions.

These patterns can mean issues like placental abruption, prolapsed cord, uterine rupture, or fetal distress.Prompt medical care is needed to ensure a healthy delivery.

Contractions After Birth

Contractions continue even after childbirth as the uterus works to shrink back down and expel any remaining tissue. These are called afterpains or postpartum cramps.

Contractions following delivery:

  • Help the uterus contract and slow bleeding.
  • May be more uncomfortable when breastfeeding due to oxytocin release.
  • Typically worst in second or later pregnancies when the uterus is worn out.
  • Can be managed with medication, warmth, and rest.
  • Usually resolve within several days but may persist for 1-2 weeks.

While afterpains can be quite uncomfortable, they serve an important purpose in the postpartum recovery process. However, speak to your provider if the pain becomes severe or you experience heavy postpartum bleeding.

When to Worry About Contractions

While contractions are a normal part of pregnancy and labor, there are some situations in which they warrant medical evaluation:

  • Contractions before 37 weeks, especially if accompanied by pelvic pressure, menstrual cramps, lower back pain, bleeding, or ruptured membranes could indicate preterm labor.
  • Painful contractions without cervical change after several hours could signal failure to progress or stalling labor.
  • Contractions slowing or spacing out more than 5 minutes apart in active labor may mean distress requiring intervention.
  • Extremely painful contractions very close together with urge to push before you are fully dilated could mean precipitous labor.
  • Prolonged, extremely frequent contractions without resting could risk uterine rupture in women with previous C-sections or uterine surgery.

The bottom line is that while contractions are expected and even helpful, certain patterns or situations do require prompt medical evaluation for the health and safety of mother and baby.


While we still do not know exactly what triggers the uterus to suddenly switch from preparing for labor to active labor itself, we do understand the important role contractions play in the birthing process. As long as they follow the normal progressive pattern, contractions are an encouraging sign that the body is moving toward vaginal delivery.

Monitoring contraction frequency, duration, and intensity helps assess the progression of labor. Abnormal contraction patterns or contractions before term require medical attention to ensure a safe labor and delivery. While labor contractions can certainly be painful, they work to achieve the wonderful goal of birthing a child.