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 18–24 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 — 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 micronised progesterone runs ~$40-80/month for ~5 months ($200-400 total course); transvaginal cervical-length measurement adds 5-10 minutes to a covered anatomy ultrasound. Cerclage is a surgical procedure covered by insurance when indicated.
Each gestational week gained between 24 and 36 weeks roughly halves the absolute risk of major neonatal disability and death; Crump et al. 2011 showed a graded all-cause mortality gradient through young adulthood, and Moster et al. 2008 documented cerebral-palsy relative risks of 78.9 at 23-27 weeks vs term. The intervention reduces lifelong disability burden in the child rather than the mother — large effect on the dyad.
Nightly vaginal insertion of a progesterone capsule from short-cervix diagnosis (around 20 weeks) through 36 weeks is a mild but persistent lifestyle addition; cerclage adds one outpatient procedure and pelvic rest for a period. Cervical-length screening itself is a single ultrasound.
Universal transvaginal cervical-length screening rests on the 1996 Preterm Prediction Study and consistent replication; vaginal progesterone for short cervix is supported by the Fonseca 2007 and PREGNANT (Hassan 2011) RCTs and confirmed by the Romero 2018 IPD meta-analysis and EPPPIC 2021. ACOG, SMFM, and WHO are aligned. Not 5: OPPTIMUM (Norman 2016) was null in the high-risk-mixed population and the PROLONG failure of 17-OHPC (Blackwell 2020) erased one limb of the framework.
For the mother-newborn pair, preventing preterm delivery substantially reduces immediate post-delivery complications (NICU separation, postpartum hemorrhage, emergency cesarean) and lets the woman experience the term-birth recovery rather than the NICU-vigil one. IPD meta-analysis showed composite neonatal morbidity/mortality dropped 41% with vaginal progesterone in short-cervix women (Romero et al. 2018).
NICU stays are a documented risk factor for maternal PTSD, depression, and partnership strain; avoiding the prolonged hospital separation and the trauma of an early delivery has a real, if secondary, effect on maternal mental health in the months after birth.