The cervix is the gatekeeper. A healthy cervix stays long and closed through the second trimester; an at-risk one shortens early. Its length on a transvaginal ultrasound at 18โ24 weeks is the single most powerful predictor of early delivery there is; 25 mm or less carries roughly six times the risk of the median 1.
Vaginal progesterone is the fix that survived. A nightly capsule tops up the local signal that keeps the uterus quiet. Started for a short cervix, it drops delivery before 33 weeks by about 40% and the worst newborn complications by the same, with no signal of harm 2. One generic capsule, a few hundred dollars for the pregnancy, no needle, no surgery.
A stitch helps a narrow group. Cervical cerclage works only when you have both a prior preterm birth and a cervix now shortening below 25 mm. With a short cervix and no such history it carries real risk and no benefit 3.
The whole thing turns on two questions asked at the right visits.
What a bought week actually buys. The mechanism is gestational time, and the curve is steep. A baby at 28 weeks faces breathing failure, brain bleeds, and lasting disability; every week the cervix holds toward 32 roughly halves that risk 4. Past 36 you're back at term outcomes: a baby that goes home with you, milestones on the normal track because the third trimester of brain-building happened inside the uterus 5.
The honest arithmetic: in a short cervix you treat about 14 women to prevent one early birth. It shifts the odds, not the outcome. But you're in the population it shifts.
The fine print โ when to skip it, and what people get wrong
"The progesterone shot works." The injectable caproate (Makena) lost FDA approval in 2023; it's a different drug from the vaginal capsule 6. "Screening catches most preterm births." It doesn't; it flags the ones progesterone helps most 7.
Progesterone is well tolerated; check first if you've had ruptured membranes or bleeding 2. It hasn't helped in twins, where the plan differs 8. Bed rest is no substitute; it can cause clots 9.
- 1Iams JM, Goldenberg RL, Meis PJ, Mercer BM, Moawad A, Das A, Thom E, McNellis D, Copper RL, Johnson F, Roberts JM (1996). The length of the cervix and the risk of spontaneous premature delivery. New England Journal of Medicine. link
- 2Romero R, Conde-Agudelo A, Da Fonseca E, O'Brien JM, Cetingoz E, Creasy GW, Hassan SS, Nicolaides KH (2018). Vaginal progesterone for preventing preterm birth and adverse perinatal outcomes in singleton gestations with a short cervix: a meta-analysis of individual patient data. American Journal of Obstetrics and Gynecology. link
- 3Berghella V, Ciardulli A, Rust OA, To M, Otsuki K, Althuisius S, Nicolaides KH, Roman A, Saccone G (2017). Cerclage for sonographic short cervix in singleton gestations without prior spontaneous preterm birth: systematic review and meta-analysis of randomized controlled trials using individual patient-level data. Ultrasound in Obstetrics & Gynecology. link
- 4Crump C, Sundquist K, Sundquist J, Winkleby MA (2011). Gestational age at birth and mortality in young adulthood. JAMA. link
- 5Moster D, Lie RT, Markestad T (2008). Long-term medical and social consequences of preterm birth. New England Journal of Medicine. link
- 6FDA (2023). FDA Commissioner and Chief Scientist Announce Decision to Withdraw Approval of Makena. link
- 7Esplin MS, Elovitz MA, Iams JD, Parker CB, Wapner RJ, Grobman WA, Simhan HN, Wing DA, Haas DM, Silver RM, Hoffman MK, Peaceman AM, Caritis SN, Parry S, Wadhwa P, Foroud T, Mercer BM, Hunter SM, Saade GR, Reddy UM (2017). Predictive accuracy of serial transvaginal cervical lengths and quantitative vaginal fetal fibronectin levels for spontaneous preterm birth among nulliparous women. JAMA. link
- 8EPPPIC Group (2021). Evaluating Progestogens for Preventing Preterm birth International Collaborative (EPPPIC): meta-analysis of individual participant data from randomised controlled trials. The Lancet. link
- 9ACOG (2021). Prediction and Prevention of Spontaneous Preterm Birth: ACOG Practice Bulletin Number 234. link
Preterm Birth Prevention
Generic vaginal progesterone runs roughly $40โ80 a month for a few months; the ultrasound is part of routine pregnancy care. Cheap relative to the stakes.
Every week a baby stays in the womb past 28 weeks roughly halves the lifelong risk of cerebral palsy and serious disability โ prevention here pays out over a whole human life.
A capsule placed at bedtime through most of the second and third trimester, plus one short extra ultrasound around 20 weeks. Mild, but it has to be done nightly.
Multiple randomized trials and patient-level meta-analyses; obstetrics societies in the U.S., U.K., and the WHO all back the core recommendation.
A timely cervix check and a nightly progesterone capsule keep more pregnancies on track to full term โ meaning a baby in your arms, not in an incubator on another floor.
Avoiding a long NICU stay also avoids the documented postpartum toll on mothers: lower rates of PTSD, depression, and strained partnerships in the year after birth.