In part 1 of this post, I was in a dilemma. Teresa was 39 weeks and 4 days by the standard calculations of her due date. Her only ultrasound, at 20 weeks, had placed her due date 9 days later, but her due date had not been changed because the rule of thumb was not to change the due date unless the ultrasound due date was 10 days different than the due date.
Now I was being pressed by the practice protocols of my group to set her up for a scheduled induction at 41 weeks. My hesitation, aside from a general avoidance of inductions, was a niggling doubt that Teresa was actually due when we thought she was. Based on her long menstrual cycles–45 days between periods was the norm for her–I calculated she was actually due about 17 days later than the due date recorded in her chart.
Teresa really wanted to avoid induction. What was the big concern about carrying a pregnancy beyond 41 weeks? Didn’t the rule used to be that you must be delivered by 42 weeks?
Two terms you will hear used by obstetricians are postdates pregnancy and postterm pregnancy. Postdates pregnancy refers to a pregnancy which is between 40 and 42 weeks gestation. Postterm pregnancy refers to a pregnancy which has reached 42 weeks or beyond.
About 3-12% of women are thought to reach 42 weeks of pregnancy without giving birth.¹ However, this number is likely high, because most postterm pregnancies are actually not postterm but simply a case of the wrong due date.²
There are known factors that increase the risk of carrying a pregnancy beyond 40 weeks³:
- primiparity (first baby)
- prior postterm pregnancy
- male gender of the fetus
- genetic factors
In fact, there is a rare condition in which a woman actually cannot go into labor on her own. Placental sulfatase deficiency is a condition in which the woman has an enzyme defect that results in prolonged pregnancy. Women with this condition rarely go into labor on their own. The condition may be related to X-linked icthyosis, a hereditary skin problem which a mother passes to her son. Many women who carry this X-linked trait are already aware of the problem because of affected males in their family.
But few women have the problem of being unable to go into labor on their own. Most women who carry a pregnancy past their due date are simply normal. In fact, research has shown that the average pregnancy lasts longer than the 266 days previously believed. You may have heard that a pregnancy lasts 280 days; this is counting from the first day of your last menstrual period, assuming a 28-day cycle. Since most women will conceive on or about day 14, the actual duration of pregnancy is said to be 266 days. However, Mittendorf et al. discovered that not only did the average pregnancy for a first-time mother (primipara) last 274 days–a full eight days longer than the previously believed 266 days–women who had given birth before also carried their pregnancies an average of three days longer than 266 days.
Despite these findings, induction at 41 weeks, rather than 42, is now being enthusiastically embraced by many obstetricians. What is the reason for this change? Studies show a small but significant increase in the risk of stillbirth beyond 41 weeks. In addition, there is n increased risk of several complications, including meconium stained fluid, neonatal acidemia (the baby not receiving enough oxygen through the umbilical cord), big babies, shoulder dystocia, and resulting birth injuries. Even more suprising, the same research found that these complications are least common at 38 weeks’ gestation, and increase weekly thereafter. This presents the interesting question of whether inductions will be increasingly pushed at ever-earlier dates.
Henci Goer presents some very interesting challenges to some of the research promoting earlier inductions. Induction of pregnancy at 41 weeks may not be the panacea it is touted to be.
And what about Teresa? Although I told her she could refuse an induction at 42 weeks (at which time I estimated she was actually 39 weeks, 4 days pregnant), the obstetrician’s dire predictions of a huge baby covered in meconium and suffering from birth injuries due to shoulder dystocia understandably frightened her into agreeing to an induction. Even though it was her second baby, the induction was an arduous 20 hours, and resulted in the birth of a healthy, 7 pound 2 ounce baby covered in vernix–a good indication that this baby was not by any means overdue.
In my next post, I’ll tell you the story of Sandy, a woman in a similar situation who made a very different choice.
1. Norwitz ER, Snegovskikh VV, Caughey AB. Prolonged pregnancy: when should we intervene?. Clin Obstet Gynecol. Jun 2007;50(2):547-57.
2. Taipale P, Hiilesmaa V. Predicting delivery date by ultrasound and last menstrual period in early gestation. Obstet Gynecol. Feb 2001;97(2):189-94.