Nov 27, 2012
What is preterm labor?
Preterm labor is labor and before 37 completed weeks of pregnancy (normally pregnancy is 40 weeks long).
Why is preterm labor undesireable?
Infants born prematurely(especially before 32 weeks) are more likely to suffer complications such as cerebral palsy, chronic lung disease, visual disturbances, and hearing impairment.
What are the symptoms of preterm labor?
Symptoms of preterm labor include:
• more than 4-6 contractions per hour for several hours in a row
• vaginal bleeding
• ruptured membranes (“broken water”)
• cervical dilation of more than 3 cm or effacement (thinning) of greater than 80%
What causes preterm labor?
There are four major pathways that lead to preterm birth:
1. over-distension of the uterus such as is caused by a multiple pregnancy (twins or more)
2. vaginal bleeding or hemorrhage between the placenta and uterine wall
3. premature activation of the pathways that lead to term labor, activation might be caused by infection.
4. the strongest risk factor for premterm labor, is a history of a previous preterm birth
What tests are used to determine whether early contractions are "false labor" or true preterm labor?
Many doctors give their patients a single injection of terbutaline when the patient presents to the hospital with contractions. If labor stops completely, it was probably a false alarm.
Additional diagnostic tests that are used to help sort out true from false labor are:
transvaginal ultrasound--cervical length of greater than 3 cm. effectively rules out preterm labor, while a cervix which is shorter than 2 cm is concerning.
A negative fetal fibronectin test is also quite helpful (fibronectin is a protein which glues the membranes to the uterine wall it is not present unless the membranes are loosening.) if the test is negative a doctor can send a patient home with confidence that she is unlikely to deliver.
What is done if true preterm labor is diagnosed?
If it appears that delivery may be imminent, magnesium sulfate is typically begun. Magnesium sulfate is given by IV in the attempt to stop true preterm labor long enough to transfer a mother to a tertiary care hospital with a NICU that is capable of caring for premature infants. Recent studies have indicated that magnesium sulfate has a protective effect of stabilizing delicate fetal blood vessels especially those in the brain--It reduces the risk of cerebral palsy and necrotizing enterocolitis.
During this time period corticosteroids are administered. Corticosteroids such as beta methasone help the fetal lungs to mature quickly--reducing the newborn's need for oxygen. Oxygen delivered in high concentrations for prolonged periods of time is associated with vision problems and hearing impairment
Who is at highest risk for preterm delivery:
• mothers with twins or higher order multiples--61% of multiple pregnancies deliver before 37 weeks.
• mothers with bacterial vaginosis have traditionally been thought to have an increased risk, but recent studies have had conflicting results—suggesting that this may not be a risk factor after all.
• mothers with a history of premature delivery in a previous pregnancy are at very high risk.
• mothers with a short cervical length--measurement of the cervix is now a standard part of an ultrasound mothers with a cervical length of less than 2 cm measured by transvaginal ultrasound are at significant risk. Mothers who have undergone treatment for cervical dysplasia such as conization or LEEP procedures are more likely to have a short cervix
• Finally, other factors associated with preterm delivery are vaginal bleeding, urinary tract infection in the current pregnancy, sexually transmitted diseases, smoking and periodontal disease
Can preterm labor be predicted and prevented before it begins?
Recent studies have shown that administration of 17 alpha hydroxyprogesterone caproate (typically used for mothers with a history of preterm birth in a previous pregnancy) statistically reduces the preterm labor.
Cervical cerclage (a suture similar to a purse string which is placed around the cervix) has also been demonstrated to reduce the number of preterm births. A cerclage can be used for women who have a history of preterm delivery and have not responded to progesterones.
The incidence of preterm birth in the United States actually increased by 20% between 1990 and 2006, (largely due to an increase in multiple gestations resulting from Assisted Reproductive Techonologies--multiple pregnancies increased by 22% over approximately the same time period) so newer and more effective strategies are needed to address preterm labor and birth.
Current research is increasing our understanding of the mechanisms that cause preterm birth. As this knowledge increases, our ability to diagnose, treat, and prevent preterm birth should improve substantially.
Additionally our ability to care for premature infants, even extremely premature infants has resulted increased survival of preterm infants and fewer disabilities.