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Does your body or the baby decide when labor starts?


The timing of when labor starts is a complex process influenced by multiple factors involving both the mother’s body and the developing baby. While the exact trigger that kickstarts labor remains unknown, we do understand many of the physiological changes and milestones that must occur for labor to begin. Let’s explore what is known about how the mother’s body and the baby work together to initiate the amazing process of childbirth.

What events need to happen for labor to start?

For most pregnancies, the following key developments must take place for labor to commence:

Fetal development

– The baby must reach full term in its development, which is typically around 40 weeks from the start of the mother’s last menstrual period. Prior to full maturity, the baby releases hormones that help maintain pregnancy.

– In the final weeks of pregnancy, the baby’s body stops making hormones that suppress labor and begins producing hormones like corticotropin-releasing hormone that help trigger contractions.

– The baby’s lungs, brain, and other organs must fully develop to survive outside the womb. The baby will signal readiness through things like surfactant production in the lungs.

Placental changes

– As the placenta ages late in pregnancy, it becomes less efficient at producing hormones like progesterone that maintain pregnancy. This shift allows other labor-inducing hormones to increase.

– The placenta begins a process called calcification where calcium builds up in preparation for the organ’s eventual delivery as the afterbirth. These changes influence releases of hormones involved in starting labor.

Cervical ripening

– The cervix needs to thin out, shorten, and dilate to allow the baby to pass through the birth canal. This process is facilitated by increased collagen breakdown, called “cervical ripening.”

– Key hormones like estrogen, prostaglandins, and oxytocin promote cervical changes leading up to labor. The cervix becomes “favorable” for birth.

– Cervical stretch receptors sense that the cervix is preparing for labor and send signals to the brain to coordinate contractions.

Uterine changes

– The uterus grows throughout pregnancy and the muscle cells that make up the uterine wall change to take on contraction capabilities.

– The uterus becomes extremely sensitive to hormones like oxytocin late in pregnancy, enabling it to start generating powerful contractions when stimulated.

– The lining of the uterus prepares for placental detachment and the uterus “practices” with small contractions, known as Braxton Hicks contractions.

Maternal triggers that influence labor’s onset

While fetal development is essential for labor to proceed, the mother’s body plays a key role in controlling the exact timing of birth. Some maternal triggers that can influence when labor begins include:

Hormone shifts

– In the final weeks of pregnancy, the mother’s hormones change to favor labor initiation. For example, progesterone declines while oxytocin and prostaglandins rise.

– The ratio of estrogen to progesterone may promote cervical ripening and enhance uterine contractions when it reaches a certain threshold.

– The mother’s hormones interact with the fetal hormones in complex ways to initiate labor. For example, fetal cortisol may help enhance maternal oxytocin.

Inflammation

– Inflammation increases gradually in the mother’s body over the course of pregnancy. This inflammatory response helps trigger and propagate labor contractions.

– Inflammatory molecules called cytokines appear crucial for cervical ripening, membrane rupture, and uterine activation.

– Infection and inflammation from sources like the vagina can enhance labor-associated inflammation locally and systemically.

Stress

– Physical and psychological stress on the mother can stimulate neuroendocrine changes that increase inflammation and promote earlier onset of labor.

– The effects of maternal stress on gestational length may be mediated through inflammatory pathways, activation of the hypothalamic-pituitary-adrenal axis, and increased oxytocin release.

– Chronic stress exposure through pregnancy can result in premature increases in inflammation and maternal cortisol that hasten labor.

Stretch and pressure

– As the uterus expands dramatically in size, the mechanical stress and pressure on the uterus, cervix, and vagina help trigger cervical and uterine changes needed for labor.

– Physical stretch on the uterine muscles enhances contractions. Stretching of nerves in the cervix prompts signals that induce labor mediators like prostaglandins.

– Increased abdominal pressure from the enlarging uterus can cause hormonal changes and direct mechanical effects like membrane rupture.

Environmental and lifestyle influences

A variety of external factors relating to the mother’s environment and lifestyle during pregnancy can also affect the timing of labor initiation, including:

Nutrition

– Malnutrition restricts fetal growth and adversely affects placental development, often delaying labor until past the due date.

– Obesity and excessive weight gain generate inflammation and alter hormone balance in ways that commonly precipitate earlier labor onset.

– Micronutrient deficiencies impair cervical ripening and uterine contractile capabilities in some cases. For example, vitamin D deficiency is linked with dysfunctional labors.

Activity level

– Regular exercise throughout pregnancy helps promote optimal inflation balance, placental and cervical health, and normal fetal growth trajectories that support on-time labor.

– High-impact exercise too close to the due date could potentially trigger premature labor through mechanisms like membrane rupture, inflammation, and uterine contractions.

– Decreased activity and muscle weakness can impair uterine contractility and delay labor. This helps explain why bedrest often prolongs pregnancy.

Sleep

– Poor sleep and sleep deprivation during pregnancy are associated with increased odds of preterm birth and shorter gestational length, likely due to effects on inflammation, hormones, and blood pressure.

– On the other hand, getting adequate sleep helps maintain normal inflammatory balance and oxytocin rhythms that promote starting labor closer to term.

– Disrupted circadian rhythms and night shift work may interfere with hormonal cues that time labor onset for optimal birthing conditions.

Toxins and pollutants

– Cigarette smoke contains chemicals that induce inflammation and alter estrogen pathways in ways linked to premature labor onset.

– Environmental pollution exposure is associated with higher rates of preterm birth, which may be explained by pollution’s effects on placental health, hormones, and inflammation.

– Toxic chemicals from some household products have been tied to earlier labor onset through unknown mechanisms that require further study.

Common tests to assess labor readiness

As a pregnancy approaches full term, women undergo certain tests to get clues about the likelihood that labor will start soon:

Fetal fibronectin test

– Fetal fibronectin is a protein released by the fetal membranes that helps the amniotic sac stick to the uterus. It detected in cervical secretions when these membranes start to detach in preparation for labor.

– A positive fetal fibronectin test starting around week 35 of pregnancy signals a higher risk of premature birth and the cervix ripening for delivery.

Bishop score

– The Bishop score grades cervical ripeness based on factors like effacement, position, consistency, and dilation. Higher Bishop scores indicate greater readiness for the cervix to start dilating for birth.

– Doctors may check the Bishop score weekly starting at 36 weeks gestation to predict the likelihood of labor or the need for induction within the next week.

Ultrasound

– Ultrasound can measure the length and funneling of the cervix to assess premature labor risk and cervical changes needed for vaginal delivery.

– Determining the fetal size and weight through ultrasound helps estimate whether the baby has undergone sufficient growth for a healthy delivery.

Contraction stress test

– This test monitors fetal heart rate changes in response to artificial contractions induced through nipple stimulation or intravenous oxytocin.

– It determines if the placenta can still provide adequate oxygen to the fetus during contractions, which predicts how the fetus may handle true labor.

When is medical induction appropriate?

If labor does not start naturally by week 42 of pregnancy, there is an increased risk of complications. In such cases, a woman’s doctor may recommend inducing labor through medical techniques like:

– Oxytocin administration through an IV drip, which stimulates uterine contractions
– Artificial rupture of membranes (breaking the amniotic sac)
– Insertion of a foley balloon catheter to help dilate the cervix
– Application of prostaglandin drugs (misoprostol) to soften and open the cervix
– Stripping or sweeping of the uterine membranes

Induction may also be warranted earlier in pregnancy if there are medical complications, such as:

– Preeclampsia or high blood pressure
– Maternal heart or kidney problems
– Restricted fetal growth
– Decreased amniotic fluid
– Placental deterioration
– Infections
– Bleeding issues like placenta previa
– Premature rupture of membranes

Choosing an appropriate labor induction time requires assessing both fetal development milestones and maternal health factors to minimize risk.

Can natural methods induce labor?

Many women attempt natural techniques at home to try to induce the onset of labor. Some methods that may provide mild stimulation include:

Nipple stimulation

– Gentle nipple stimulation prompts uterine activity by activating oxytocin release through a neural reflex. It should be done with caution and not over-used due to oxytocin’s potent effects.

Sexual intercourse

– Semen contains prostaglandins that help promote cervical ripening, while female orgasm causes uterine contractions. However, the mild stimulation of sex only seems to help ripen a cervix that is already favorably prepped for delivery.

Walking and exercise

– Light exercise can enhance blood flow and stimulate prostaglandin release to favor cervical changes without causing excessive inflammation. Staying active also promotes optimal fetal positioning.

Acupuncture

– Acupuncture may help induce labor by reducing stress, increasing relaxation, altering inflammatory pathways, and influencing natural opioid levels. But clinical evidence is still inconclusive.

Castor oil

– Castor oil contains ricinoleic acid that can stimulate intestinal cramping to indirectly prompt uterine contractions. However, it also causes unpleasant gastrointestinal side effects like diarrhea.

Herbal supplements

– Some herbs like red raspberry leaf and evening primrose oil contain compounds that may promote cervical ripening and uterine relaxation. But their true efficacy and safety are uncertain.

When should you call your doctor about labor signs?

It is a good idea to contact your healthcare provider if you notice any of the following possible signs of impending labor:

– Lightening – Baby drops lower into the pelvis/abdomen
– Changes in vaginal discharge – More mucus-like or bloody
– Diarrhea – Prostaglandins can stimulate the bowels
– Cramps – Contractions may feel like menstrual or gastrointestinal cramping at first
– Pelvic pressure – Baby’s head presses down
– Back pain – Lumbar discomfort from the uterus contracting
– Rupture of membranes – Water breaking from the amniotic sac

Calling when labor symptoms first arise allows your provider to advise you on next steps and determine if you should come into the hospital to be examined for dilation and effacement of the cervix. Don’t wait until contractions are very painful before seeking guidance.

Conclusion

The initiation of labor is influenced by an incredible synergy between maternal and fetal physiology. While science has uncovered many of the hormones, inflammatory factors, signaling pathways, and structural changes involved, the unique timing of when a woman’s body declares it’s time for her baby’s birth remains somewhat mysterious. What we do know is that labor’s onset requires achieving a threshold where the uterus is primed to contract, the cervix is soft and ready to open, and the baby has completed all necessary development. While mothers can implement natural techniques thought to promote labor onset, working with your doctor is crucial for determining when induction may provide important health benefits for both you and your child. Paying attention to your body’s signs and calling your provider when you notice any symptoms of labor brewing ensures you have the safest, healthiest birthing experience possible.