Preterm birth is when your baby is delivered before 37+0 weeks gestation. Spontaneous preterm birth is when regular painful contractions causing the cervix to dilate leading to preterm labour and delivery. The rate of preterm birth is about 7% in Ireland. The risk is increased to 14% in women who have had 2 LLETZ procedures or a cone biopsy, which can cause the cervix to shorten earlier in pregnancy than it should. For women who have had one LLETZ procedure the risk is not thought to be increased and is still about 7%. Women with a previous spontaneous preterm birth have the highest risk of preterm birth. It is estmiated that the risk is 30% following one previous preterm birth and 45-60% following two or more previous preterm births.
So how can we predict preterm birth? It has been shown that 30% of women with symptoms of preterm labour are admitted to hospital but only 5-10% of these women will deliver within 7 days. This can lead to anxiety and unnecessary admissions to hospital. Transvaginal (internal) ultrasound measurement of the length of your cervix can be used to determine if you are at risk of preterm birth. In those with risk factors discussed above, it is helpful. A short cervix is when the measurement is <25mm at <28 weeks gestation. In women without risk factors but with symptoms of threatened preterm labour, a cervical length <25mm has a positive predictive value of 30% and a negative predictive value of 89% for delivery within 7 days. Other test used to predict preterm birth use vaginal swabs including fetal fibronectin and partosure.
Fetal fibronectin has a positive predictive value of 24% and a negative predictive value of 89% for delivery within 7 days. Thus, a negative result is reassuring but a positive result is less reliable.
So how do we prevent preterm birth? Screening high risk women with an ultrasound measurement of cervical length should take place between 14 and 16 weeks for the first measurement and then if the measurment is normal at least one other measurment should be performed at 18-22 weeks gestation. Some screening clinics for high risk women will offer cervical length scans every 2 weeks from 14 to 28 weeks gestation. If the cervical length is <25mm at <24+0 weeks gestation you can be offered a cervical cerclage (stitch). Progesterone has also been used to prevent preterm birth, either vaginal progesterone daily or proluton injection once a week from 17 weeks until 34 weeks gestation.
So if your baby is delivered preterm how can we reduce the risks of prematurity? Steroids in pregnancy for women with threatened preterm labour reduce the chance of respiratory distress syndrome (lung problems), intraventricular haemorrhage (brain bleeds), necrotising enterocolitis (gut problems) and death when a baby is born preterm. They are given by intramuscular injection, 2 doses, 24 hours apart. They work best if delivery occurs >48 hours but within less than seven days after administration. They may be effective if your baby is delivered between 23 and 34 weeks gestation, and may even be of benefit up to 37 weeks gestation. A drip called magnesium sulphate has been used to protect a baby’s brain if you deliver preterm between 23 and 32 weeks gestation. Breastfeeding is very important for a preterm baby as it will reduce the risk of serious gut problems for the baby as well as all the other benefits it confers.