View Question
Q: Birth date versus estimated due date statistics ( Answered ,   1 Comment )
 Question
 Subject: Birth date versus estimated due date statistics Category: Family and Home > Parenting Asked by: jifl2-ga List Price: \$15.00 Posted: 20 Jul 2003 21:22 PDT Expires: 19 Aug 2003 21:22 PDT Question ID: 233200
 ```I've been wondering what the distribution of babys' births are relative to their Estimated Due Date (EDD). For example how many babies are born before 37th week (pre-term), at the start of the 37th week, end of 37th week, start of 38th week etc. up to after 42nd week. Ideally it would be some study with a daily granularity so I can see the cumulative chance of when a baby is likely to arrive by a certain date! I understand that first babies are normally later, so there may well be different statistics for first and subsequent babies. I also found out that 10% are born after the 42nd week and 5% born on due date, but I'm looking for a greater level of detail than that. I would have thought that somewhere Out There is some study with what surely should be common knowledge in midwifery circles. A quick google myself didn't find anything though. Thanks.```
 ```Dear jifl2-ga, Thanks for the opportunity to address this interesting question! As I had two "late" deliveries myself, I had previously done some research on the subject. First of all, there is a question of how to determine a due date. There are several different methods and all of them can return different results. Occasionally the results can be inaccurate, since many women do not ovulate on the 14th day of their cycle. In fact, there was even a recent study done that shows some women ovulate multiple times during one menstrual cycle: -Yahoo News - Women May Ovulate More Than Once a Month: Study http://story.news.yahoo.com/news?tmpl=story&u=/nm/20030708/wl_canada_nm/canada_health_ovulation_col_1 Although this study has not yet been independently verified, the implications could be fairly important in the ob/gyn sphere. So how does one determine a due date? The method most people are familiar with is called Naegele’s Rule. Add 280 days to the date of the last menstrual period, or 266 days to the date of ovulation, if known. This was an arbitrary rule developed by Franz Naegele in 1812 according to his belief that pregnancies _should_ last ten lunar months. Secondly is Mittendorf's formula, based on his observation that most first time, or primaparous, women gave birth 8 days later than the due date calculated by Naegele's Rule. Women who had given birth before, or multiparous women, on average gave birth 3 days past their Naegele due date. Black women tended to give birth 8.5 days earlier than white women. Therefore, the Mittendorf formula is: (LMP – 3 months) + 15* Days = Due Date * Add 10, rather than 15, if mother is non-white, or multiparous -PubMed: The Length of Uncomplicated Human Gestation http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=2342739 -Calculating Due Dates http://www.transitiontoparenthood.com/ttp/birthed/duedatespaper.htm According to a German doctor in the early 1800s, there is even a formula for determining gestation length in the Talmud: "The regular pregnancy from the day of fertilization until the delivery has a duration of 270 days or 9 months, each month counted as 30 days." -Due Date (Frank Peinemann) http://frank.peinemann.bei.t-online.de/medicine/duedate.htm As you can see, there is no one definitive way to tell when a baby is due. Some babies just need to "bake" longer, and some are perfectly happy coming out sooner. Each method can vary the due date by days, and therefore skew the delivery statistics that you are interested in. Now to the meat of the matter. You want to know the distribution of babies that are born full-term during the 37th, 38th, 39th, etc. weeks. Unfortunately there is nothing out there with raw data per day - everything seems to be compiled per week. Most all information I have seen follows the pattern of a bell chart with the peak around 40 weeks, so you can safely assume that the highest percentage numbers given up until 40 weeks are for the _end_ of the week, and after 40 weeks are for the _beginning_ of the week. Understandably this involves some smoothing, but from my research I have seen no large-scale multinational studies done, so undoubtedly the figures you get will be close but not exact. "I have found multiple references that state that about 6% of babies are pre-term, and that 4-14% of babies would be born after 42 weeks (Odutayo, and others), if it were not for interventions..." -Calculating Due Dates, ibid. The Calculating Due Dates ( http://www.transitiontoparenthood.com/ttp/birthed/duedatespaper.htm ) page also has a graph with percentages broken down per week, from extrapolated data from different sources. It states that births break down as follows (from eyeballing the bar graph): Before 37 weeks: 4% 37 weeks: 5% 38-39 weeks: 10% 39-40 weeks: 20% 40-41 weeks: 35% 41-42 weeks: 20% Over 42 weeks: 10% (the numbers do not add up to 100%, but unfortunately data is not given for the graph) The page also gives a distribution of data from births in Alaska from 1990-1998, and notes that there may be a racial difference in the Alaskan study because "Native infants on average weigh slightly more than non-Native infants". Very preterm babies, under 33 weeks, were 1.9%. Preterm babies, 33-36 weeks, were 9.4%. Term babies, 37-41 weeks, were 81.6%. Postterm babies, 42 weeks and over, were 9.0% A midwife states: "I have just finished our 1995 stats (I know, I know....I'm a little slow, but I do all the data and we had 479 women (with 487 babies...6 sets of twins, 2 sets of triplets) that year. 217 primips... Of these 479 women: 140 (29.2%) delivered between 40-41 weeks 105 (21.9%) delivered between 41-42 weeks 27 (5.6%) delivered between 42-43 weeks 1 went >43 completed weeks" -Gentlebirth Postdates archive http://gentlebirth.org/archives/postdates.html There is a Canadian study contrasting gestational age at birth between 1972 and 1986 here: -eCMAJ -- Joseph and Kramer 161 (11):1409 http://www.cmaj.ca/cgi/content/full/161/11/1409 with a line graph of percentage of live births vs. gestational age in weeks here: http://www.cmaj.ca/cgi/content/full/161/11/1409/F129 Another Canadian study of term births is available here: -Recent Trends in Fetal and Infant Outcomes Following Post-term Pregnancies http://www.hc-sc.gc.ca/pphb-dgspsp/publicat/cdic-mcc/22-1/a_e.html with the following breakdowns: 1985-1987 37 weeks - 4.76% 38 weeks - 12.65% 39 weeks - 19.54% 40 weeks - 37.27% 41 weeks - 13.71% 42 weeks - 4.96% 43+ weeks - 0.41% 1992-1993 37 weeks - 5.69% 38 weeks - 14.36% 39 weeks - 21.56% 40 weeks - 32.44% 41 weeks - 14.65% 42 weeks - 3.40% 43+ weeks - 0.13% I assume the missing percentage applies to births before 37 weeks. It is interesting to note that due dates are normally the beginning of the 40th week. According to these studies, you only have a 29-37% chance of delivering in your 40th week! I hope this answer suits your needs, and I thank you heartily for giving me the opportunity to research something so interesting! If you have any other questions or concerns, please use the Clarification Request feature so that I may help further. Search strategy used: Henci Goer's book, The Thinking Woman's Guide to a Better Childbirth, makes note of the discrepancy in calculating due dates and the inclination for induction by the medical community for post-term babies. This provided me with a starting point for my search. She related the Mittendorf study in the appendix. Google searches: "length of uncomplicated human gestation" "estimated due date statistics" "gestational age distribution" "distribution percentage human gestation weeks"```
 jifl2-ga rated this answer: ```A brilliant answer. I feel like I can't give the full 5 because there there isn't the granularity I was after, but it seems that if there is any study out on the web it all, it may not be google-able. Nevertheless the answer was very complete, thank you!```
 ```I asked this same question a while back: http://answers.google.com/answers/main?cmd=threadview&id=110577 At that time, the consensus was that there was no scientific answer. Anyway, my results was that she came 1 day early.```