Signs of preterm labor

By trimester

Weeks 1–13

1st trimester

Preterm labor doesn't apply in the first trimester by definition: it specifically covers 20 to 37 weeks, so anything before that falls under different guidance. Contact your provider about any bleeding or severe cramping regardless of how early it is.

Weeks 14–27

2nd trimester

The earliest and most serious preterm labor starts after 20 weeks, in the second trimester. Watch for the same signs (frequent contractions, fluid leakage or bleeding, pelvic pressure, low back pain, cramping) and call right away rather than waiting.

Weeks 28–birth

3rd trimester

Most preterm labor happens in the third trimester, especially the late-preterm window of 34 to 36 weeks. The same warning signs apply: more than six contractions an hour, fluid leakage or bleeding, or new pelvic pressure or backache are reasons to be evaluated the same day.

What does the evidence show for signs of preterm labor?

• The warning signs of preterm labor include contractions every 10 minutes or more often, fluid leakage or bleeding from the vagina, pelvic pressure, low back pain, and cramping that can come with diarrhea. 1
• More than six contractions in an hour is the specific threshold to call your provider about or go in to be evaluated, rather than waiting it out at home. 1
• Preterm labor is defined by regular contractions plus cervical change between 20 and 37 weeks of pregnancy, and worldwide, about 10% of births happen preterm. 2
• Providers evaluating possible preterm labor do a pelvic exam to check whether the cervix is thinning or opening, along with ultrasound and contraction monitoring, and sometimes a fetal fibronectin test, which is most useful for its reassuring negative result showing delivery within a week is unlikely. 3
• There's currently no reliable way to predict who will go into preterm labor, and the underlying causes are not fully understood, which is why the warning signs above are worth calling about rather than waiting to see if they pass. 4

When should I call my provider about signs of preterm labor?

More than six contractions in an hour, fluid leaking or gushing from the vagina, vaginal bleeding, a sudden increase in pelvic pressure, or new low back pain and cramping between 20 and 37 weeks are the specific signs that call for contacting your provider or going in to be evaluated the same day, not at the next scheduled appointment. Providers can't reliably predict who will go into preterm labor, which is why acting on these signs early matters more than trying to wait them out. Whatever the pattern, any symptom that feels beyond your usual range is a reason to call: obstetric teams expect these questions and would rather hear early than late.