Preventing Preterm Birth
/Today’s episode returns back to the world of preterm labor and birth. For the management of active preterm labor or premature prelabor rupture of membranes (PPROM), see our previous episode on the subject. For today, ACOG PB 130 and PB 142 are essential reading, though we’ll also get into the recent controversy on this subject with the PROLONG trial (excellent summary by our friends at the OBG Project here).
Screening: Risk Factors and Sonographic Cervical Length
Spontaneous preterm birth is responsible for significant neonatal morbidity and mortality, and thus its prevention is paramount. Unfortunately, we don’t have a lot of great tools for prediction or prevention right now, but history is the starting point to identify those at risk. Prior history of spontaneous preterm birth carries a 1.5-2.0-fold risk of recurrence. Other associative risk factors include chronic illness or inflammatory states; acute illness such as UTI/pyelonephritis; periodontal disease; history of cervical excisions biopsy (LEEP or CKC); low prepregnancy weight (BMI < 20); smoking or substance use; and short interpregnancy interval. There are likely a number of other environmental factors we just don’t have good data about.
An additional risk factor that often leads to confusion amongst trainees is the shortened cervical length, as measured transvaginally. The indication for intervention depends on the history.
For women with no prior history of spontaneous PTB: a single measurement of the cervical length transvaginally may be considered if abnormal on abdominal ultrasound.
If the cervical length is <20mm, vaginal progesterone supplementation (200 mg PV) can be offered.
For women with prior history of spontaneous preterm birth: q2week measurements of cervical length vaginally can be a screening tool between 14 or 16-24 weeks.
These patients should already be considered for IM progesterone starting at 16 weeks gestation (more on that soon!).
Ultrasound-indicated cerclage should be considered if the cervical length is <25mm and the prior PTB occurred at <34 weeks.
No screening intervention has been shown to be of benefit in multifetal gestations.
Intervention: Using Progesterone
As we’ve briefly mentioned above, progesterone for PTB prevention is in two flavors: vaginal and intramuscular. As shown above, the only ACOG-approved recommendation for vaginal progesterone is for an incidentally-identified short cervix < 20mm prior to 24 weeks, and no prior history of spontaneous PTB.
Intramuscular progesterone (17-hydroxyprogesterone, or 17-OHP for short) is recommended for all women who have a prior history of spontaneous PTB prior to 36 weeks. It is administered as a weekly injection starting at 16 weeks through delivery or 36 weeks.
17-OHP has come under some attention recently due to the publication of the PROLONG trial. Prior to this, the MFM U study authored by Meis demonstrated 17-OHP as an effective option for prevention of recurrent spontaneous PTB. As we note in the podcast, there are some important differences between the trials, which should help you counsel your patients. At this time, ACOG and SMFM still endorse its use as originally marketed, though an FDA advisory panel has issued a non-binding recommendation to remove 17-OHP from the market. We should have an update sometime this summer, so stay tuned!
Intervention: Cerclage Placement
Cerclage placement is another tool for prevention of preterm birth. There are three primary indications:
ACOG PB 142
It’s important to commit these to memory! The challenge clinically for both history and examination-indicated cerclage are teasing out “painless cervical dilation,” versus true preterm labor. Some physicians may even recommend amniocentesis prior to a procedure to determine if an infection is present before placing a cerclage. It’s also not clear in the history- or sonographic-indicated cerclage classifications if the addition of 17-OHP helps to prevent PTB.