Contractions can start as early as the second trimester of pregnancy, around 16 to 20 weeks. These early contractions are almost always Braxton Hicks contractions, which are practice tightenings of the uterus that don’t signal labor. True labor contractions typically begin at or after 37 weeks, though preterm labor can start earlier.
Braxton Hicks: The Earliest Contractions
Braxton Hicks contractions are the first type of contraction most pregnant people experience. They can begin in the second trimester, though many women don’t notice them until the third trimester. Some never feel them at all. These contractions are your uterus rehearsing for delivery, tightening for 30 to 60 seconds and then releasing. They’re typically mild, irregular, and felt mainly in the front of the abdomen.
Several everyday triggers can set off Braxton Hicks contractions. Dehydration is one of the most common: when your body is low on fluids, the uterus becomes more irritable, leading to cramping and irregular tightening. Physical activity, a full bladder, sex, and even the baby’s movements can also bring them on. Drinking a glass or two of water, changing positions, or resting for a few minutes will usually make them fade.
How Braxton Hicks Differ From Real Labor
The single biggest difference is pattern. True labor contractions come at regular intervals, and those intervals get shorter over time. Braxton Hicks contractions are scattered. They might come twice in ten minutes, then not again for an hour. They have no predictable rhythm, and they don’t get closer together.
Intensity matters too. Real labor contractions steadily get stronger as time goes on. Braxton Hicks contractions stay about the same strength or may even start strong and then weaken. True labor contractions also last longer, typically 60 to 90 seconds each, and the pain usually starts in the back before wrapping around to the front. With Braxton Hicks, the sensation is generally limited to the front of the abdomen.
The simplest test is movement. If you change positions, take a walk, or lie down and the contractions stop, they’re almost certainly Braxton Hicks. True labor contractions continue no matter what you do.
Irritable Uterus: Frequent but Not Labor
Some women develop a pattern of frequent, regular contractions well before their due date that don’t fit neatly into the Braxton Hicks category. This is sometimes called an irritable uterus. The contractions can be stronger and more frequent than typical Braxton Hicks, and they don’t always respond to rest or hydration, which makes them harder to distinguish from preterm labor on your own.
The key feature is that these contractions don’t cause any changes to the cervix. The cervix doesn’t dilate or thin out, so the pregnancy isn’t progressing toward delivery. It’s an uncomfortable condition, but it’s not dangerous on its own. If you’re having contractions that feel more intense or frequent than you’d expect, your provider can check your cervix to rule out actual labor.
When Contractions Signal Preterm Labor
Preterm labor is defined as labor that begins before 37 completed weeks of pregnancy. It’s broken into subcategories: extremely preterm (before 28 weeks), very preterm (28 to 32 weeks), and moderate to late preterm (32 to 37 weeks). The earlier labor starts, the greater the health risks for the baby.
A useful threshold to keep in mind: six or more contractions in one hour before 37 weeks is a reason to call your provider or go to the hospital. Other warning signs that can accompany preterm contractions include a change in vaginal discharge, pelvic pressure that feels like the baby is pushing down, low back pain that doesn’t go away, and any fluid leaking from the vagina.
Dehydration is one of the most preventable triggers. When dehydration goes uncorrected, irregular uterine cramping can progress to genuine preterm labor. Staying well hydrated throughout pregnancy reduces this risk significantly.
How Providers Assess Early Contractions
If you show up with contractions before 37 weeks, your provider’s first goal is determining whether your cervix is changing. A cervical exam and sometimes an ultrasound measurement of cervical length give them a direct answer. But there’s also a swab test that checks for a protein called fetal fibronectin in your vaginal fluid. This protein acts like a biological glue between the amniotic sac and the uterine lining, and its presence after about 22 weeks can signal that the body may be preparing for delivery.
A negative result on this test is particularly reassuring. It’s very accurate at predicting that labor won’t happen in the next 14 days, which can save you from an unnecessary hospital stay. A positive result doesn’t guarantee early delivery, but it does mean your provider will monitor you more closely and may give treatments to support the baby’s lung development or slow contractions down.
Why the Uterus Contracts Before Labor
Your uterus is a muscle, and like any muscle it can tighten and relax on its own. Throughout pregnancy, the hormone oxytocin binds to receptors on the uterine muscle and triggers contractions. Early in pregnancy, these receptors are relatively few and not very sensitive, which is why early contractions tend to be mild and infrequent.
As pregnancy progresses, rising estrogen levels make those receptors more sensitive and more numerous. By the final weeks, the uterus responds much more strongly to oxytocin. Oxytocin also stimulates the release of other compounds that soften and thin the cervix, preparing it for delivery. This gradual buildup is why contractions naturally become more frequent and powerful as you approach your due date. The early Braxton Hicks tightenings you feel months before delivery are essentially your uterus responding to low levels of oxytocin before the system is fully ramped up.