The Truth About Due Dates in Pregnancy

At your very first prenatal visit, you receive a due date. But how many of us were told that this is just an estimate? Let’s talk about due dates in pregnancy, due dates based on conception, due dates based on ovulation, and so much more! 

Trigger warning: Stillbirth and miscarriage.

It’s exciting and surreal to imagine the date our baby is expected to come. But, your midwife or OB, yourself, and those around you need to know that this is just an estimate.

Currently, in our medical system, women are already being offered and sometimes pushed into induction of labor without medical reason. Further, often forgetting the risks of induction and only focusing on not going past one’s due date. Is this the right way to go about due dates in pregnancy?

What is a Due Date in Pregnancy?

The estimated due date (EDD) is the date that spontaneous onset of labor is expected to occur.

How to Calculate Due Date in Pregnancy?

For a long time, the most common, mainstream method of calculating due dates in pregnancy was Naegeles Rule.

Naegeles Rule, also referred to as last menstrual period (LMP), calculated an estimated due date by adding 280 days (9 months and 7 days) to the first day of the last menstrual period (LMP).

This method calculates a due date based on ovulation by relying on accurate recall of the mothers first day of their last menstrual period, assumes you have the average 28 day cycle, and that you ovulated on day 14 of that cycle.

This doesn’t always happen this way, especially if you don’t have the average 28 day cycle.

Use of the last menstrual period (LMP) to establish the due date may overestimate the duration of the pregnancy, meaning they would think the pregnancy is further along than it actually is, by possibly more than 2 weeks [1-3]. This leads to unnecessary induction, dysfunctional labor and cesarean section, and resultant neonatal and maternal morbidity.

Due dates in pregnancy based on conception are used when the date of conception is known precisely, such as with in vitro fertilization, the estimated due date is calculated by adding 266 days to the date of conception.

How Accurate Are Due Dates in Pregnancy?

Doctors started using ultrasound in the 1970s. Soon after, ultrasound measurement replaced using the last menstrual period (LMP) as the most reliable way to determine gestational age [4].

Substantial evidence shows that ultrasounds done in early pregnancy, by measuring CRL (crown-rump length of the baby), are more accurate than using LMP to date a pregnancy [5].

Researchers found that people who had an early ultrasound to estimate the due date were less likely to be labeled as “post-term” and have an unnecessary induction.

Also, in a large study with more than 17,000 pregnant women in Finland, researchers found that ultrasound at any time between 8 and 16 weeks was more accurate than the LMP. When ultrasound was used instead of LMP, the number of “post-term” pregnancies decreased from 10.3% to 2.7% [6].

Why is Last Menstrual Period Less Accurate than Using Ultrasound?

LMP is often less accurate than ultrasound to define gestational age because it can have these problems: [7-9]

• Women can have irregular menstrual cycles, or cycles that are not 28 days (28 days is only the average length of menstrual cycles)
• Women might be uncertain about the date of their LMP
• Many women do not ovulate on the 14th day of their cycle
• The embryo may take longer to implant in the uterus for some people
• Research indicates that some people are more likely to recall a date that includes the number 5, or even number, so they may inaccurately recall that the first day of their LMP has one of these numbers in it.

What is the Best Time to Use Ultrasound to Determine Due Dates in Pregnancy?

One study in 2013 looked at ultrasound scans at <7 weeks, 7-10 weeks, 11-14 weeks, 14-19 weeks, and 20-27 weeks [10].

The researchers found that the most accurate time to perform an ultrasound to determine the gestational age was 11-14 weeks. About 68% of the women gave birth ±11 days of their estimated due date when it was calculated with ultrasound at 11-14 weeks.

In this study, the accuracy of ultrasound significantly declined starting at about 20 weeks.

For more information about the benefits, risks, and accuracy of ultrasound for fetal weight, check out The Truth About Baby Ultrasounds.

Can Due Dates Change During Pregnancy?

In the Listening to Mothers III survey, one in four mothers (25%) reported that their midwife or OB changed their estimated due date based on a late pregnancy ultrasound.

For 66% of the mothers, the estimated due date was moved to an earlier date, while for 34% of the mothers, the date was moved to a later date [11].

Ultrasounds after 20 weeks, and especially in the third trimester, are not as accurate because they are comparing the size of your baby to a “standard” sized baby. Although, all babies are about the same size in early pregnancy, this does not remain true as the pregnancy progresses.

Therefore, if a baby during this time is larger than the “standard” size being compared, this does not accurately mean the baby is older than what was already estimated. This can lead to an unnecessary induction (and increased risk of a cesarean birth) for being “post-term”.

On the other hand, if a baby is measuring smaller than the “standard”, this does not mean they are younger than you and your midwife or OB thought. This could be risky if a baby is experiencing growth restriction, as growth restricted babies have a higher risk of stillbirth towards the end of a pregnancy.

Further, the American College of Obstetricians and Gynecologists states that due dates should only be changed in the third trimester in very rare circumstances [9].

How do Due Dates in Pregnancy Help?

Estimated due dates in pregnancy are helpful to give the mother, the family, and the midwife or OB an idea how long their pregnancy will be and when the mother will go into labor.

What is Full Term for Pregnancy?

One very important study from 2001 looked at the length of pregnancy in 1,514 healthy women whose estimated due dates using LMP perfectly matched with the estimated due dates from their first trimester ultrasound [12].

The researchers found that 50% of all women giving birth for the first time gave birth by 40 weeks and 5 days, while 75% gave birth by 41 weeks and 2 days.

On the other hand, 50% of the women who have given birth before, gave birth by 40 weeks and 3 days, while 75% gave birth by 41 weeks.

In this study, full term for pregnancy was at least 5 days longer than the traditional due dates in pregnancy (using Naegele’s rule/LMP) in first time mothers.

For mothers who have given birth before, full term pregnancy was at least 3 days longer than the traditional due dates in pregnancy.

Another study looking at full term pregnancy

In 2013, an analysis looked at 125 healthy women to evaluate the normal length of pregnancy [8].

The median time from the first day of the last menstrual period to birth was 285 days (or 40 weeks, 5 days after the last menstrual period).

The length of pregnancy ranged from 36 weeks and 6 days to one person who gave birth 45 weeks and 6 days after the last menstrual period. The 45 weeks and 6 days sounds really long… but this particular person actually gave birth 40 weeks and 4 days after ovulation. Her ovulation did not fit the normal pattern, so we know her LMP due date was not accurate.

Ultimately, the researchers found that:

• 10% gave birth by 38 weeks and 5 days after the LMP
• 25% gave birth by 39 weeks and 5 days after the LMP
• 50% gave birth by 40 weeks and 5 days after the LMP
• 75% gave birth by 41 weeks and 2 days after the LMP
• 90% gave birth by 44 weeks and zero days after the LMP

Remember though, this study used LMP and some of the participants did not ovulate on the 14th day of their cycles.

If these statistics are adjusted for their ovulation, you’ll see:

• 10% gave birth by 36 weeks and 4 days after ovulation
• 25% gave birth by 37 weeks and 3 days after ovulation
• 50% gave birth by 38 weeks and 2 days after ovulation
• 75% gave birth by 39 weeks and 2 days after ovulation
• 90% gave birth by 40 weeks and zero days after ovulation

Interestingly, women who had embryos that took longer to implant were more likely to have longer pregnancies [8]. Meanwhile, women who had a late rise in progesterone had a pregnancy that was about 12 days shorter.

What Things Can Make Your Pregnancy Last Longer?

Genetics is the most important predictor of a longer pregnancy.

One large study looked at more than 475,000 Swedish births and found that genetics has an incredibly strong influence on your chance of having a birth after 42 weeks [9]. The researchers found that:

• If you have had a post-term birth before, you have 4.4 times the chance of having another post-term birth with the same partner
• Provided that you have had a post-term birth before, and then you switch partners, you have 3.4 times the chance of having another post-term birth with your new partner
• Also, if your sister had a post-term birth, you have 1.8 times the chance of having a post-term birth

Other factors that may make your pregnancy more likely to go longer include:

• Higher body mass index before you get pregnant [8, 13-14]
• Higher weight gain during pregnancy [14]. If you’d like to learn more about where weight gain in pregnancy comes from and exercise tips for pregnancy, check out: How to Exercise During Pregnancy for a Better Labor!
• Longer time between when you ovulated and when your pregnancy implanted [8]
• Older maternal age [8, 13]
• Heavier birth weight of the mother [8] (Fun fact: I was 9.9 lbs when I was born!)
• Higher education level of the mother [13]
• If it’s your first time being pregnant [13]
• If you’re pregnant with a male baby [13]
• Your mother had a post-term birth [13]
• The baby is measuring small by ultrasound at 10-24 weeks [15]
• Experiencing environmental stress towards the end of pregnancy (at 33-36 weeks) [16]

What Are the Risks of Going Past Your Due Date?

Before getting into these statistics, especially the ones regarding the risks for mothers, keep in mind that these are not based on mothers and babies who received the midwifery model of care.

If you’re not sure what the difference is, I highly recommend that you check out: 21 Undeniable Benefits of the Midwifery Model of Care.

risks for mothers:

• The risk of chorioamnionitis (infection of the membranes surrounding the baby) is lowest at 37 weeks (0.16%) and increased every week after that to a high of 6.15% at ≥ 42 weeks [17]. One thing that increases this risk is vaginal exams. Were the physicians potentially performing more exams because they were antsy about the mother being later in her pregnancy?
• Risk of endomyometritis (infection of the uterus) was lowest at 38 weeks (0.64%) and increased every week after that to a high of 2.2% at ≥ 42 weeks [18]
• The risk of having a placenta abruption (placenta separates prematurely from the uterus) was lowest at 37 weeks (0.09%), and increased every week to a high of 0.44% at ≥ 42 weeks [17]
• Risk of preeclampsia was lowest at 37 weeks (0.4%) and highest at 40 weeks (1.5%), after which the risk did not change [17]
• The risk of postpartum hemorrhage was lowest at 37 weeks (1.1%) and increased almost every week to a high of 5% at 42 weeks [19]
• Risk of cesarean in women who have never had a cesarean before increased from 14.2% at 39 weeks to a high of 25% at ≥42 weeks [18]
• The risk of a cesarean for non-reassuring fetal heart rate (in women who have never had a cesarean before) was lowest at 37-39 weeks (13.3-14.5%) and reached a high of 27.5% at 42 weeks [19]
• Risk of receiving forceps or vacuum assistance increased from 14.1% at 38 weeks to a high of 18.5% at 41 weeks [18]

risk for infants:

• The risk of moderate or thick meconium increased every week starting at 38 weeks, and peaked at ≥42 weeks (3% at 37 weeks, 5% at 38 weeks, 8% at 39 weeks, 13% at 40 weeks, 17% at 41 weeks, and 18% at >42 weeks) [18]
• Neonatal intensive care unit (NICU) admission rates were lowest at 39 weeks (3.9%) and rose to 5% at 40 weeks and 7.2% at ≥42 weeks [18]
• The odds of having a low 5-minute Apgar score went up starting at 40 weeks and increased each week until ≥42 weeks (exact numbers were not reported) [18]

Stillbirth Meaning

Before we talk about stillbirth, it’s important to know “what is stillbirth?”.

The definition of stillbirth is death in utero due to any cause after 20 weeks gestation.

Stillbirth meaning is different from a miscarriage, as a miscarriage is loss of a baby before the 20th week of pregnancy.

Stillbirth Causes

These factors do not necessarily cause stillbirth, but they may increase the risk of stillbirth.

In general, these factors may include:

• Belonging to an ethnic group at increased risk for stillbirth [20]. Keeping in mind that racism, including the effects of prejudice and institutional racism, can increase the risk of poor outcomes, including stillbirth, in certain populations [23]
• If this is your first pregnancy [20-22]
• Having fewer than four prenatal visits or no prenatal care [21-22]
• A body mass index (BMI) over 25 to 30 [20-22]
• Smoking [22]
• Pre-existing diabetes [20, 22]
• Pre-existing hypertension [22]
• Older maternal age (≥40 years)
• Not living with a partner [20]
• History of previous stillbirth [20]
• Being pregnant with multiples [20]

However, As many as a third of all stillbirths that take place before labor have no known cause [24]. To read more about theories of unexplained stillbirth, read this article here. 

How Are Stillbirth Rates Measured?

Regarding stillbirth, researchers use a formula to calculate stillbirth rates.

Interestingly, the risk of stillbirth decreases throughout pregnancy, until it reaches a low point at 37-38 weeks. After which the risk started to rise again.

Because the risk of stillbirth starts to go up again at and past due dates in pregnancy (40, 41, and 42 weeks), some researchers argue that although 40 weeks and 3-5 days may be the physiological length of pregnancy, 40 weeks may be the functional length of a pregnancy.

By receiving this information about due dates in pregnancy, you get to make an informed decision on what you feel is best for you and your baby!

How Might a Midwife Determine Due Dates in Pregnancy?

There can be situations where a woman might want to greatly limit or completely avoid technology in her perinatal care.

Or, a woman might start her prenatal care late and not know when her last menstrual period was.

In either of these cases, if the mom and baby are seemingly healthy, the midwife can estimate that the fundal height (the distance between your pubic bone to the top of your uterus) often equals the gestational age of the baby.

For example, if you’re 32 weeks pregnant, your fundal height should measure should measure about 32 cm.

Therefore, the midwife can use this knowledge to estimate due dates in pregnancy.

Also, it’s normal for the fundal height to measure ±2 cm from the gestational age. But if the fundal height measures beyond this normal range, you might be advised to receive further testing such as a Non-Stress Test (NST) or a Biophysical Profile (BPP).

Furthermore, this is a great example of one reason I love and highly recommend kick counting as a way to check-in on your baby’s well-being!

Questions or Comments on “The Truth About Due Dates in Pregnancy”?

Have any questions or comments regarding due dates in pregnancy? Please leave them below👇🏻

Talk soon, mama!

– Katelyn Lauren

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DISCLAIMER: This post is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding you or your baby’s health. Please read my Medical Disclaimer for more info

References:

[1]  Kramer MS, McLean FH, Boyd ME, Usher RH: The validity of gestational age estimation by menstrual dating in term, preterm and postterm gestations. JAMA 1988; 260:3306. PMID:3054193 

[2] Savitz DA, Terry JW Jr, Dole N, Thorp JM Jr, Siega-Riz AM, Herring AH.Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol. 2002 Dec;187(6):1660- PMID:12501080

[3] Wilcox M, Gardosi J, Mongelli M, et al. Birth weight from pregnancies dated by ultrasonography in a multicultural British population. BMJ. Sep 4 1993;307(6904):588-91.PMID:8401014 

[4] Morken, N. H., Klungsoyr, K., et al. (2014). Perinatal mortality by gestational week and size at birth in singleton pregnancies at and beyond term: a nationwide populationbased cohort study. BMC Pregnancy Childbirth 14: 172.

[5] Whitworth M, Bricker L, Mullan C. (2015). Ultrasound for fetal assessment in early pregnancy. Cochrane Database Syst Rev (7): CD007058.

[6] Taipale, P. and Hiilesmaa, V. (2001). Predicting delivery date by ultrasound and last menstrual period in early gestation. Obstet Gynecol 97(2): 189-194.

[7] Savitz, D. A., Terry, Jr., J. W., et al. (2002). Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol 187(6): 1660-1666.

[8] Jukic, A. M., Baird, D. D., et al. (2013). Length of human pregnancy and contributors to its natural variation. Hum Reprod 28(10): 2848-2855.

[9] American College of Obstetricians and Gynecologists (2017, Reaffirmed 2019). Committee Opinion No. 700. Methods for Estimating the Due Date. 129, e150-154.   

more references:

[10] Khambalia, A. Z., Roberts, C. L., et al. (2013). Predicting date of birth and examining the best time to date a pregnancy. Int J Gynaecol Obstet 123(2): 105-109.

[11] Declercq, E. R., Sakala, C., Corry, M. P., et al. (2013). Listening to MothersSM III: Pregnancy and Birth.New York: Childbirth Connection.

[12] Smith, G. C. (2001a). Use of time to event analysis to estimate the normal duration of human pregnancy. Hum Reprod 16(7): 1497-1500.

[13] Oberg, A. S., Frisell, T., et al. (2013). Maternal and fetal genetic contributions to postterm birth:familial clustering in a population-based sample of 475,429 Swedish births. Am J Epidemiol 177(6): 531-537.

[14] Halloran, D. R., Cheng, Y. W., et al. (2012). Effect of maternal weight on postterm delivery. J Perinatol 32(2): 85-90.

[15] Johnsen, S. L., Wilsgaard, T., et al. (2008). Fetal size in the second trimester is associated with the duration of pregnancy, small fetuses having longer pregnancies. BMC Pregnancy Childbirth 8: 25.

[16] Margerison-Zilko, C. E., Goodman, J. M., et al. (2015). Postterm birth as a response to environmental stress : The case of September 11, 2001. Evol Med Public Health 2015(1): 13-20.

[17] Caughey, A. B., Stotland, N. E., et al. (2003). What is the best measure of maternal complications of term pregnancy: ongoing pregnancies or pregnancies delivered? Am J Obstet Gynecol 189(4): 1047-1052. 

[18] Caughey, A. B. and Musci, T. J.  (2004). Complications of term pregnancies beyond 37 weeks of gestation. Obstet Gynecol 103(1): 57-62.

[19] Caughey, A. B. (2007). Measuring perinatal complications: methodologic issues related to gestational age. BMC Pregnancy Childbirth 7: 18.

more references:

[20] Stillbirth Collaborative (2011). Association between stillbirth and risk factors known at pregnancy confirmation. JAMA 306(22): 2469-2479.

[21] Huang, D. Y., Usher, R. H., et al. (2000). Determinants of unexplained antepartum fetal deaths. Obstet Gynecol 95(2): 215-221.

[22] Flenady, V., Koopmans, L., et al. (2011). Major risk factors for stillbirth in high-income countries: a systematic review and meta-analysis. Lancet 377(9774): 1331-1340.

[23] Giscombe, C.L. and Lobel, M. (2005). Explaining disproportionately high rates of adverse birth outcomes among African Americans: the impact of stress, racism, and related factors in pregnancy. Psychol Bull 131(5): 662-83.

[24] Warland, J. and Mitchell, E. A. (2014). A triple risk model for unexplained late stillbirth. BMC Pregnancy Childbirth 14: 142.