Ans. is c, i.e. 2.5 cm.Preterm labor (PTL): Preterm labor is defined as labor (regular, painful frequent uterine contractions causing progressive effacement and dilatation of cervix) occurring before 37 completed weeks of gestation."Risk of preterm birth increases markedly when the cervix is less than 2.5 cm. This measurement has been widely accepted as the threshold to define the risk of premature birth. The possibility of preterm delivery when the cervix is <25 mm is 17.8%. This risk is significantly greater than the normal risk, and hence these women require additional diagnostic tests and special care." --Fernando Arias 3/e, p 229, 230Also Know:Most common cause of preterm labor is idiopathic followed by infection like urinary tract infection, vaginal infections etcMost common organisms responsible for preterm labor:Ureoplasma urealyticum and Gardenerella vaginum causing bacterial vaginosis.Diagnosis of Preterm labor is by: --Dutta 9/e, p 294-Regular uterine contractions with or without pain (at least 4 in every 20 min or 8 in 60 min).-Dilatation (> 2 cm) and effacement (80%) of cervix.-Length of cervix (measured by TVS) < 2.5 cm and funneling of internal os.-Pelvic pressure, backache and or vaginal discharge or bleeding.Thus for detecting preterm labor -TVS should be done and cervical length measured. Besides this fetal fibronectin if present in vaginal/cervical secretions before 37 weeks indicates preterm labor.Fetal fibronectin:It is a fetal glycoprotein.Normally it is found in the cervicovaginal discharge before 22 weeks and again after rupture of membranes.If detected in cervicovaginal secretions prior to rupture of membranes, it indicates disruption of the maternal-fetal interface and may be predictive of impending preterm labor.It is measured by ELISA and a value equal to or exceeding 50 ng/ml is considered positive and predictive of preterm delivery.When the test is negative it reassures that delivery will not occur within next 14 days.Remember: Fetal fibronectin equal to or more than 50 ng/ml and cervical length <2.5cm on TVS are the best predictors especially in a woman with a prior history of preterm birth.