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Postterm pregnancy is when a woman has not yet delivered her baby after 42 weeks of gestation, two weeks beyond the typical 40-week duration of pregnancy. Postmature births carry risks for both the mother and the baby, including fetal malnutrition, meconium aspiration syndrome, and stillbirths. After the 42nd week of gestation, the placenta, which supplies the baby with nutrients and oxygen from the mother, starts aging and will eventually fail. Postterm pregnancy is a reason to induce labor.Definitions
The management of labor and delivery may vary depending on the gestational age. It is common to encounter the following terms when describing different time periods of pregnancy.
Postterm – ≥ 42 weeks + 0 days of gestation (> 293 days from the first day of last menstrual period, or > 13 days from the estimated due date)
Late term – 41 weeks + 0 days to 41 weeks + 6 days of gestation
Full term – 39 weeks + 0 days to 40 weeks + 6 days of gestation
Early term – 37 weeks + 0 days to 38 weeks + 6 days of gestation
Preterm – ≤ 36 weeks + 6 days of gestationBesides postterm pregnancy, other terminologies have been used to describe the same condition (≥ 42w+0d), such as prolonged pregnancy, postdates, and postdatism. However, these terminologies are less commonly used to avoid confusion.Postterm pregnancy should not be confused with postmaturity, postmaturity syndrome, or dysmaturity. These terms describe the neonatal condition that may be caused by postterm pregnancy instead of the duration of pregnancy.Signs and symptoms
Because postterm pregnancy is a condition solely based on gestational age, there are no confirming physical signs or symptoms. While it is difficult to determine gestational age physically, infants that are born postterm may be associated with a physical condition called postmaturity. The most common symptoms for this condition are dry skin, overgrown nails, creases on the baby's palms and soles of their feet, minimal fat, abundant hair on their head, and either a brown, green, or yellow discoloration of their skin. Doctors diagnose postmature birth based on the baby's physical appearance and the length of the mother's pregnancy. However, some postmature babies may show no or few signs of postmaturity.Baby
Reduced placental perfusion – Once a pregnancy has surpassed the 40-week gestation period, doctors closely monitor the mother for signs of placental deterioration. Toward the end of pregnancy, calcium is deposited on the walls of blood vessels, and proteins are deposited on the surface of the placenta, which changes the placenta. This limits the blood flow through the placenta and ultimately leads to placental insufficiency, and the baby is no longer properly nourished. Induced labor is strongly encouraged if this happens.
Oligohydramnios – Low volume of amniotic fluid surrounding the fetus. It is associated with complications such as cord compression, abnormal heart rate, fetal acidosis, and meconium amniotic fluid.
Meconium aspiration syndrome – Respiratory compromise secondary to meconium present in infant's lungs.
Macrosomia – Excessive birth weight, estimated fetal weight of ≥ 4.5 kg. It can further increase the risk of prolonged labor and shoulder dystocia.
Shoulder dystocia – Difficulty in delivering the shoulders due to increased body size.
Increased forceps-assisted or vacuum-assisted birth – When postterm babies are larger than average, forceps or vacuum delivery may be used to resolve the difficulties at the delivery time, such as shoulder dystocia. Complications include lacerations, skin markings, external eye trauma, intracranial injury, facial nerve injury, skull fracture, and rarely death.Mother
Increased labor induction – Induction may be needed if labor progression is abnormal. Oxytocin, a medication used in induction, may have side effects such as low blood pressure.
Increased forceps assisted or vacuum assisted birth – operative vaginal deliveries increase maternal risks of genital trauma.
Increased Caesarean birth – Postterm babies may be larger than an average baby, thus increasing the length of labor. The labor is increased because the baby's head is too big to pass through the mother's pelvis. This is called cephalopelvic disproportion. Caesarean sections are encouraged if this happens. Complications include bleeding, infection, abnormal wound healing, abnormal placenta in future pregnancies, and rarely death.A 2019 randomized control trial of induced labor at 42 or 43 weeks was terminated early due to statistical evidence of "significantly increased risk for women induced at the start of week 43". The study implies clinical guidelines for induction of labor no later than at 41 gestational weeks.Causes
The causes of post-term births are unknown, but postmature births are more likely when the mother has experienced a previous postmature birth. Due dates are easily miscalculated when the mother is unsure of her last menstrual period. When there is a miscalculation, the baby could be delivered before or after the expected due date. Postmature births can also be attributed to irregular menstrual cycles. When the menstrual period is irregular it is difficult to judge the moment of ovulation and subsequent fertilization and pregnancy. Some postmature pregnancies may not be postmature in reality due to the uncertainty of mother's last menstrual period. However, in most countries where gestation is measured by ultrasound scan technology, this is less likely.Monitoring
Once a pregnancy is diagnosed postterm, usually at or greater than 42 weeks of gestational age, the mother should be offered additional monitoring as this can provide valuable clues that the fetal health is being maintained.
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