Figuring Your Due Date

by Kmom

Copyright 1996-1997 Kmom@Vireday.Com. All rights reserved.

Why Is Accurate Dating Important?

Figuring due dates accurately is tricky and has very important implications for your pregnancy. If your due dates are figured incorrectly, you may be pressured into all kinds of unnecessary interventions at 'term' when you are not really at term at all. A common result is to be pressured into a pitocin induction of labor, which when done on an unripe cervix has a very high failure rate, can be quite painful, and often ends in a C-section for "failure to progress." In addition, babies who are induced due to incorrect dating may be born earlier than nature intended, and can have immature lungs and other problems, needing special care. It is in Mother's and Baby's best interest to date the pregnancy correctly; this procedure is of critical importance!

Naegele's Rule (40-week pregnancy, counting from last menstrual period) generally works fairly well as a rule of thumb for the majority of pregnancies. However, large women know from experience that one size does NOT fit all. There are exceptions that must be considered, but many physicians rigidly apply the same dating rules for every woman anyway. If you fit some of the descriptions below yet your doctor does not seem flexible in regard to dating, this may be an early sign that you need to seek another health provider ASAP. If your due dates are set incorrectly, your AFP tests (alpha-fetoprotein tests for birth defects) may come back with a false positive (indicating a possible problem---see the section on Large Women and Prenatal Testing), you may be pushed into unnecessary fetal testing at 'term' and may well be pressured strongly into inducing labor with pitocin too early in the pregnancy, yet at a point when you are physically and emotionally very vulnerable to suggestion. If your cycles are about 28 days, quite regular, and this is not your first child, your physician's dating is probably fine. If your cycles are longer or irregular (as some large women's cycles are) or if this is your first child, the due date your physician has given you may be off, setting you up for all kinds of problems. Study up on the issues involved and strike up a dialogue with your physician.


Summary - Who Should Further Examine Their Due Dates?

If this is your first pregnancy, then your pregnancy will quite likely be longer. Usually, the first-time mom averages 8 days beyond her 40-week due date, if she is white and a private-care patient. (Women of color tend to have slightly shorter pregnancies, as do women not under private-insurance care, for whatever reason.) Your doctor will probably want to keep the traditional 40-week-from-LMP dating rather than change the date, but should be more flexible about not pressuring you into interventions before 42 weeks, assuming all is otherwise well. It is quite NORMAL for a first-time mom to go at least a week 'overdue' and pressure for interventions should probably not begin during that time! Try and sound out your health provider about when they think interventions should start to be considered; if they sound very interventionist you may wish to consider switching to a different care provider, depending on your philosophy. DON'T WAIT till you are 40 weeks along to consider the issue!

If your cycles are longer than 28 days, then your pregnancy may be longer. You may need to add another week or two onto your totals. Some doctors are quite good about taking this into account, but unless you can document that you KNOW when you ovulate or when you conceived, many doctors will still arbitrarily set your date according to the 28-day cycle, possibly setting you up for early interventions, a difficult labor and perhaps even a C-section (worst-case scenario). It is in your best interest to dialogue with your physician about this concern; you may be able to come up with a compromise date or use ultrasound to confirm dating. However, if your physician seem overly rigid in dating, it may be time to seek an alternate provider. Charting before future pregnancies is an excellent idea for you to consider.

If your cycle is quite irregular, you may have great difficulty dating your pregnancy. If you have records showing when you ovulate (either by charting or by ovulation predictor kits), then these can be used to help determine a more accurate due date. If you do not have any idea of when you typically ovulate or if it varies significantly from month to month, then your doctor will probably need to use multiple ultrasounds during the pregnancy to date your pregnancy via measurements of the fetus. These are most accurate before about 20 weeks or so. You will also probably need more testing at term to confirm whether baby is doing OK, due to the possibility that dating may be off. If you are not already pregnant, it is an excellent idea to start charting your temperature and mucus to avoid this difficult problem. If you are already pregnant, you will need to rely on your doctor's expertise and ultrasound testing to help you set a reliable due date.


Common Terms and Abbreviations

Discussions of dating can get confusing, so it is important to clarify some terms and abbreviations ahead of time. Also note that discussions of pregnancy dating applies to NORMAL pregnancies, not those in which significant complications occur. Sometimes, early induction of labor is medically justified.

LMP = Date of Last Menstrual Period

EDC = Estimated Date of Conception

ACOG = American College of Obstetrics and Gynecology

Primiparas (primip) = a woman's first pregnancy

Multiparas (multip) = a woman who has already had one 'successful' pregnancy; a woman who has miscarried before is usually still considered a primip.

BBT = Basal Body Temperature; when a woman takes her temperature first thing in the morning every day of her cycle in order to establish when ovulation/conception occur. Often, cervical mucus is also used in conjunction with BBT to confirm dating. More information about how to do this can be found in the book, Taking Charge of Your Fertility by Toni Weschler.

NFP = Natural Family Planning, which uses BBT and cervical mucus charting in order to help avoid or achieve pregnancy. Many Catholics utilize this method of birth control, but many non-Catholics use it too. It is not the same as the Rhythm Method, and it does have a high degree of accuracy when used correctly and with proper training. There are other similar methods as well, such as the Billings Method. For more information, contact

GD = Gestational Diabetes. Larger women are somewhat more at risk for this, and it can impact how long your pregnancy is 'allowed' to go, depending on your level of glucose control, your baby's well-being, and the standards that your personal health provider uses in delivering women with GD. It used to be standard that a woman with GD be delivered between weeks 38 and 39; now women with good control and no outside insulin are usually 'allowed' to go to 40 weeks/term. Some practitioners even 'allow' women to go to 41 or even 42 weeks, providing all tests are fine and good control is maintained. Dating is a critical issue for women with GD.


Naegele's Rule (Standard Pregnancy Dating Method)

With Naegele's Rule, a pregnancy lasts 40 weeks after LMP, or 280 days. This allows for 2 weeks of 'pregnancy' in the beginning when you are not really pregnant (the time between LMP and ovulation/fertilization). If considering from conception instead of LMP, pregnancy in this system lasts 38 weeks or 266 days. It is extremely important to know whether a system is referring to dating from LMP or from conception. In the system that most American physicians use, dating is done from LMP. Whenever a number is used that does not specify whether it is from LMP or from conception, it should be assumed that it refers to LMP.

This 40-week system presupposes that every woman's cycle is 28 days long and that she ovulates on day 14. If you have regular 28-day cycles, this is a good dating system for you. If your cycles are significantly longer, it is not. Remember, first pregnancies will probably run over this date by about a week. If this is a subsequent pregnancy, your pregnancy will be closer to the predicted 40 weeks, on average. But of course, YOUR case may be different!

To predict your due date using Naegele's Rule, take the date of your last LMP and add 7 days. Then count back 3 months to get your due date. For example, if your LMP was June 22, adding 7 days comes up with a date of June 29; subtracting 3 months then comes up with a due date of March 29th.

(This example is drawn from Kmom's second pregnancy, by the way, for comparison purposes with other prediction methods. So by Naegele's Rule, this pregnancy should be due on about March 29th, and concerns about inducing and testing will start being applied by most doctors a week or so later, around April 5th or so. As you will see, this dating is incorrect---for MY situation.)


Prem's Rule (Good for Longer Cycles But Only If Ovulation Date is Known)

Dr. Konald A. Prem at University of Minnesota Medical School has another formula that works for women who ovulate later than day 14. The problem is that you have to be charting your basal body temperature to know how to figure it. Since Prem is involved in Natural Family Planning, the women he deals with know when they ovulate from keeping detailed BBT and cervical mucus charts. When they ovulate, their temperature rises significantly (overall thermal shift) and cervical mucus changes. From knowing this plus when they last had intercourse, an estimated date of conception can be figured and a more accurate due date calculated.

In Prem's rule, the due date is taken from the first day of overall thermal shift minus 7 days, then plus 9 months. This has a high rate of accuracy for women whose cycles are longer than 28 days and who can document their temperature shifts/ovulation. It may be of some limited use to women who are not charting but who have regular periods that run longer than 28 days and know about on which day ovulation usually occurs. It can also be of use if you are irregular but have not yet conceived and can start charting. If your cycles are irregular and you are not charting, this is not a good system for you.

Because my cycles average about 35 days and because my husband and I were charting in order to achieve pregnancy (it was effective, it only took us one try!), we used Prem's rule to convince my OB to alter my due dates. Based on my overall thermal shift, she agreed to set my due date a bit over a week later, but only after quite a bit of discussion. This is a BIG deal for someone with GD, where they often don't 'allow' you to go over your due date at all, or at most by a week. So if I hadn't had charts to prove it, my doctor would've put my due date on about March 29 or so, and I would've been pressured into an induction or C-section by about April 6, my true due date. So now with accurate charts my 'clock' starts ticking on April 6, with pressures starting around April 13 or so for induction and such. A big difference! (Footnote: I found a different care provider who would not automatically require an induction at 41 weeks due to GD. We then found out that my GD did NOT recur this time so I will be 'allowed' to go over till 42 weeks anyhow! Midwives are terrific!)

Knowing that your cycles run longer can also make a HUGE difference to your AFP test results, if you choose to take that test. Both of my AFPs have come back as "low" (danger of Down's Syndrome) because of misdated gestational age. For example, when we took the AFP test in the first pregnancy, baby's gestational age was one week less than her LMP age because I ovulate a week later than most women. Thus her AFP levels were lower than her LMP age seemed to indicate and we were told that the baby was at risk for Down's Syndrome. We were scared to death and let ourselves be talked into an amnio (which was awful!) as a result. We had about a month of not knowing about her status because of this problem, which was one of the worst times of our life. This time we documented our conception, so we were able to have them refigure the preliminary results based on baby's correct gestational age, and this time the results were normal. But having correct dates can certainly alleviate a lot of potential stress!


"The Length of Uncomplicated Human Gestation." (1990 Study)

There is also evidence that pregnancy lengths varies according to race and how many children a woman has had. White, private-care women apparently carry a bit longer, and it is well-known that first pregnancies usually last longer than subsequent pregnancies. The following is a summary of a 1990 abstract ("The Length of Uncomplicated Human Gestation.") from an Obstetrics and Gynecology journal (authors from Harvard and Boston area). In order to understand his numbers, you need to remember that the Naegele's 280 day rule for pregnancy is based on LMP; he is subtracting 14 days from this for a total of 266 days from conception (assuming day 14). He's showing that this 266 day total is incorrect, especially for primips. Comments in {brackets} are mine for clarity. Note also that they are excluding pregnancies with complications.

"...we found that uncomplicated spontaneous labor pregnancy in private-care white mothers is longer than Naegele's rule predicts. For Primiparas, the median duration of gestation...was 274 days {compare 274 to the 266 doctors use currently--8 days later!} For Multiparas, the median duration...was 269 days...Thus, this study suggests that when estimating a due date for private-care white patients, one should count back 3 months from the first day of the last menses, then add 15 days for primiparas or 10 days for multiparas, instead of using the common algorithm for Naegele's rule."

So a pregnancy for a private-care 1st-time white mom is 8 days longer than usually predicted; for a private-care multip it's closer to the usual total.

Remember, though, that this does NOT take into account differences in cycle length. So if your cycles run longer than 28 days, you still need to

Kmom's Opinions and Summary

Anyhow, just a few things to think about. I strongly oppose the pressures healthy moms face for inductions and such when going 'past due' (post-mature is a different thing) since the 280/266 day rule (N's rule) is not well-supported by research. I think that the strong pressure to intervene with pitocin and Artificial Rupture of Membranes/AROM when a woman is a week past due' technically but just reaching her correct due date according to much research is a strong reason for the very high 25% c-section rate in our country. But of course, doctors are under very strong economic and litigation pressures, and choosing a date is not an easy or clear thing. They deserve some sympathy!

The implications for women reading this are whether your due date is correct, whether you should consider induction and when, and (for women who have not yet conceived) whether you should be charting if you are trying to become pregnant. If your due date is predicted by your LMP using the usual 280 day rule, then it's probably a week short if you are a primip. If your cycles are *also* longer than usual or irregular, then your dates may well be off by up to 2 weeks. However, ultrasounds up to week 20 or so can help place the dating, but babies that run bigger than average can complicate even this. See how confusing this whole mess can be? I sympathize with the doctors for needing to set a date, but I think moms are well-advised to be VERY cautious in allowing inductions and interventions. When possible, let your baby choose his own birthday. Just one opinion!

Copyright 1997 KMom@Vireday.Com. All rights reserved. No portion of this work may be sold, either by itself or as part of a larger work, without the express written permission of the author; this restriction covers all publication media, electrical, chemical, mechanical or other such as may arise over time.

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