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Spontaneous Preterm Birth
Preterm birth is the commonest cause of perinatal death in babies without congenital abnormalities. You will review this topic in detail in Case 9. There are also other demographic features that are associated with preterm birth. Maori women have a higher risk compared to Caucasian women (RR 1.6- could be related to increased smoking rates) and the risk is doubled for underweight women, teenagers and for lower socio-economic group women. Women who use recreational drugs (including Marijuana) have increased risk with of preterm birth.
Infections in pregnancy can also increase the risk. There are flags that you may find in the booking gynaecology and obstetric history that may also alert you to an increased risk for preterm birth such as previous cone biopsy (RR 2-3), uterine abnormality (RR 7) and dilation of the cervix (for early miscarriages or terminations) also increases the risk of pre-term birth. A previous midtrimester loss or preterm delivery will also alert the clinician to a 3-4 fold increase in risk of a preterm birth in this pregnancy.
ACTIVITY-What are the modifiable risk factors for preterm birth (PTB)?
One reason for antenatal screening is so that we can look for risk factors and then see if some of them can be modified. Have a look at the discussion we have had so far. What are some modifiable risk factors for PTB? You should be able to think of at least 3.
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Smoking-smoking cessation programmes
- Drug and alcohol cessation programmes
Treatment of bacteriuria, chlamydia, and bacterial vaginosis (BV) -this assumes that you have tested for these at the booking visit (note there does not appear to be a benefit in treating BV in asymptomatic women who do not have risk factors for spontaneous preterm birth)
- Treatment of UTIs
Avoiding the risk factors for cervical incompetence -eg HPV vaccine and regular smears to prevent need for cone biopsy for high grade lesions, good contraception to prevent need for surgical terminations of pregnancy for unwanted pregnancies
- Influenza vaccination
- Optimising medical conditions that may lead to a risk of preterm birth (e.g. high blood pressure, diabetes)
- Healthy weight gain in pregnancy
Giving progesterone may help some women
Over the last 15 years, research has looked at whether progesterone can help prevent preterm birth in women with risk factors. The evidence at present suggests that progesterone is recommended for asymptomatic women with a shortened cervix (<25mm) on transvaginal ultrasound in the second trimester. It should also be considered for women with a history of spontaneous preterm birth. Progesterone has not beeen proven to be effective for multiple pregnancies in women with a history of preterm birth. More information can be found in the RANZCOG 2017 statement Progesterone: Use in the second and third trimester of pregnancy for the prevention of preterm birth.
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