Investigations
1st investigations to order
non-stress cardiotocogram
Test
This is not a specific test for premature labour but an initial step to establish well-being of the fetus.
Result
detection of fetal heartbeat
tocography
Test
The frequency of contractions can be documented on a trace. More than one contraction every 10 minutes is more significant, and premature labour may be more likely.
Result
>1 contraction every 10 minutes
transvaginal ultrasound of the cervix
Test
Women who present with contractions prior to 37 weeks' gestation with a closed cervix should have a vaginal examination and transvaginal assessment (where available) of the cervical length. Cervical length can indicate the likelihood of imminent delivery, as cervical lengths under 2 cm are associated with much higher risks of delivery (>60%).[55] Transvaginal assessment of cervical length has greater predictive value if performed in combination with the fetal fibronectin test.[28]
Result
significant if cervical length <2 cm
cervico-vaginal swab for fetal fibronectin
Test
All women presenting with preterm contractions between 24 and 35 weeks' gestation, who are not in advanced labour (cervical dilation <3 cm), should be considered to have a cervico-vaginal swab for fetal fibronectin. False positives can occur in the presence of vaginal bleeding, cervical cerclage, and cervical manipulation. In a systematic review, 20% of those with a positive fibronectin test deliver within 1 week compared with only 1% with a negative test.[96] A bedside test can also quantify fetal fibronectin, where the concentration correlates with risk: levels under 10 nanograms/mL are very low risk, and levels over 200 nanograms/mL give a 45% chance of delivering in 2 weeks.[107] The fetal fibronectin test has greater predictive value if performed in combination with transvaginal assessment of cervical length.[28][99][100] Its use is associated with reduced resource usage.[108] This test is also valuable in asymptomatic women who are at high risk of premature labour.[109]
Guidance from NHS England recommends quantitative fetal fibronectin as it can be used in asymptomatic women and has better sensitivity than alternatives.[105]
Result
positive
FBC
Test
Women with threatened premature labour should have an FBC. Results are non-specific, but an elevated WBC is indicative of infection, and haemoglobin level should be checked in cases of suspected antepartum haemorrhage.
Result
decreased haemoglobin with antenatal haemorrhage; raised WBC count in presence of infection
CRP
Test
Non-specific, but recommended as part of the infection screen for all women with threatened premature labour.
Result
elevated in presence of infection
urine dipstick
Test
Women who present with threatened premature labour should have a urinalysis for proteinuria, leukocytes, and nitrites as part of the infection screen as a possible cause of premature labour. Proteinuria may be seen in pre-eclampsia.
Result
positive in presence of infection
urine microscopy, culture, and sensitivity
Test
If urine dipstick is positive for leukocytes or nitrites, a midstream urine specimen should be sent for microscopy, culture, and sensitivities.
Result
positive growth in presence of infection
high vaginal/rectal swab
Test
A high vaginal and rectal swab can be taken to screen for group B streptococcus.[106] The presence is associated with premature labour, but treatment does not affect outcome.
Result
positive in presence of group B streptococcus
Investigations to consider
microscopy of vaginal fluid
Test
Diagnosis of preterm prelabour rupture of membranes (PPROM) is usually clinical. Vaginal fluid is examined after drying. False negatives are common.
Result
amniotic fluid crystallises and may leave a fern-leaf pattern
Kleihauer blood test
Test
This blood test, evaluated in the laboratory for the presence of fetal cells, is usually used to detect fetal-maternal haemorrhage in rhesus-negative mothers in order to calculate how much anti-D is needed. A positive Kleihauer test in threatened premature labour may indicate retroplacental bleeding as a cause of the preterm contractions, although the clinical utility of this is uncertain and not all clinicians perform this test.
Result
positive with fetal-maternal haemorrhage.
urine toxicology screen
Test
A urine toxicology screen may be indicated if recreational drug use is suspected as cocaine can induce premature labour. It should be noted that such a screen is not definitive for the detection of recreational drugs.
Result
positive if cocaine has been taken
insulin-like growth factor binding protein-1 (IGFBP-1) test
Test
This is a bedside immunochromatographical dipstick test that detects the presence of the phosphorylated form of IGFBP-1 in cervical secretions. It is an alternative to the fetal fibronectin test. A positive test result is shown as two blue lines - a control line and a test line in the result area. If only the control line has appeared after 5 minutes, the test result is negative. This has also been used to confirm preterm prelabour rupture of membranes (PPROM) with sensitivities of over 90% and specificities over 98%.[102]
The UK National Institute for Health and Care Excellence recommends that IGFBP-1 may be used to help guide management for suspected PPROM if amniotic fluid is not visible, but suggests evidence is insufficient for routine use to diagnose premature labour in women with intact membranes.[73][104]
Result
positive
placental alpha microglobulin-1 (PAMG-1)
Test
May be used to determine the risk of delivery in women with threatened premature labour who have intact membranes.[103] This test is performed by taking a low vaginal swab and inserting the test strip into a small tube that indicates a positive or negative result.
The UK National Institute for Health and Care Excellence recommends that PAMG-1 may be used to help guide management for suspected preterm prelabour rupture of membranes (PPROM) if amniotic fluid is not visible, but suggests evidence is insufficient for routine use to diagnose premature labour in women with intact membranes.[73][104]
Result
positive
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