"average" length of pregnancy

Renee W

"average" length of pregnancy

I have just finished (and quite enjoyed) the Thinking Woman's Guide to a Better Birth. I read in the chapter on Induction of Labor that the "average" length of pregnancy for a first-time mom is 40 weeks + 8 days, while for a mom with prior births it is 40 weeks + 3 days.

I am due with #2 in November, however, #1 was delivered via scheduled C-section due to frank breech presentation. My question is whether the "average" length for multiparous women depends on whether the first birth was vaginal. I could not find the reference to the 1990 study that is mentioned in the section on "overdue" pregnancy. I have heard that VBACs when the C-section was performed in the absence of labor are very much like first-time deliveries (e.g. longer pushing stage, for example) but I have not read any studies on the topic.

I am also interested in learning what the error bars are on the average length estimates.



Henci Goer

RE: "average" length of pregnancy
(in response to Renee W)

As far as I know, the timing of the onset of labor is not affected by whether the first delivery was vaginal or via cesarean. What I can tell you is that research finds that the length of labor tends to be more like a first labor than like labor in a multiparous woman, as this study and this study attest. This would make sense. Many women have a first cesarean because their labor progressed more slowly than their care provider was willing to allow and so the VBAC population would skew toward women who labor more slowly. It is also possible that women having VBAC labors may be more anxious--especially if those around them are unsupportive and doubtful of their ability to birth vaginally--and this too could inhibit labor progress. The lesson to be learned is that women and their care providers should be patient in VBAC labors. Unfortunately, though, at least one study has found that women were given less time in labor, not more, if their prior cesarean was for delayed progress than if it were for other reasons such as breech or fetal distress, and this study found that one-third of women who had repeat cesareans for delayed progress or failed induction had them before 5 cm dilation, which means many women were still in latent labor. These findings highlight the importance of finding a care provider who encourages VBAC, not just agrees to it. If that is not possible, forewarned is forearmed. Women should be prepared to refuse a repeat cesarean based only on exceeding an arbitrary time limit.

As for your question on the length of pregnancy, a lot of water has flowed under the bridge since I wrote TWGBB. I have additional data on pregnancy length which can be found in my new book for professionals Optimal Care in Childbirth: The Case for a Physiologic Approach (c) 2012, co-authored by Amy Romano, co-author. Here is the relevant excerpt from the book:

How Long Is Normal Pregnancy?

The 40-week due date was set by fiat in the early 1800s by a German obstetrics professor who declared that pregnancy lasted 10 lunar months (10 months of 4 weeks each) from the beginning of the last menstrual period, hence the eponymous Naegele’s rule (Baskett 2000). Two modern-day studies of pregnancy duration with reliable dates found that the true median length (half of births before, half after) is longer and varies by parity, falling at 284 days in nulliparous women and 282-283 days in parous women (Bergsjo 1990; Smith 2001). A third study also found that after adjustment for other factors, nulliparous women beginning labor spontaneously averaged pregnancy durations three days longer than similar multiparous women (Mittendorf 1993). One of the studies establishes that a 41 week (287 d) definition of “postterm” is not physiologic. Investigators found that, depending on age and parity, at least 25% of women had not given birth by 41 completed weeks, (287 d) whereas by 42 weeks only 10% remained undelivered (Bergsjo 1990). Ten percent is a defensible definition for postterm; 25% or more is not, and there is more: by curtailing pregnancy, induction confounds duration. The same study found a dip in gestational length in pregnancies ending in December, which investigators attributed to elective inductions before the Christmas holidays (Bergsjo 1990), and an analysis of U.S. data between 1989 and 1998 found that large increases in the percentages of induced labors at all periods of gestation over the decade had shifted the distribution curve of pregnancy duration to the left (MacDorman 2002). None of these studies establishes the length of normal pregnancy in healthy women because the inclusion of preterm births and complicated pregnancies also pulls the distribution curve to the left (Mittendorf 1993). The only study we have on pregnancy duration in uncomplicated pregnancy at term reports a median of 288 d in nulliparous women and 283 d in parous women, longer than the other two studies, but still within their possible ranges (Mittendorf 1990).

Baskett TF, Nagele F. Naegele's rule: a reappraisal. BJOG 2000;107(11):1433-5.

Bergsjo P, Denman DW, 3rd, Hoffman HJ, et al. Duration of human singleton pregnancy. A population-based study. Acta Obstet Gynecol Scand 1990;69(3):197-207.

MacDorman MF, Mathews TJ, Martin JA, et al. Trends and characteristics of induced labour in the United States, 1989-98. Paediatr Perinat Epidemiol 2002;16(3):263-73.

Mittendorf R, Williams MA, Berkey CS, et al. The length of uncomplicated human gestation. Obstet Gynecol 1990;75(6):929-32.

Mittendorf R, Williams MA, Berkey CS, et al. Predictors of human gestational length. Am J Obstet Gynecol 1993;168(2):480-4.

Smith GC. Use of time to event analysis to estimate the normal duration of human pregnancy. Hum Reprod 2001;16(7):1497-500.

~ Henci 

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