Due Date Changes in Pregnancy: What to Expect and Why They Happen

If pregnancy is a journey (and it is!), then your estimated due date (EDD) is the destination. From scheduling appointments with your OBGYN and learning how do doctors calculate due date or using an online pregnancy due date calculator, to ensuring your little one is growing on track, virtually every element of prenatal care revolves around your predicted due date.

So, what happens when that destination shifts a little to the right or left?

More than ¼ of women see a change in estimated due date during pregnancy. If your estimated date shifts within 14 days before or after your original EDD, it’s largely considered normal. These changes are even more likely towards the end of your journey—but if they happen earlier, it could indicate something important about the health of your pregnancy and future baby (or babies).

Below, we break down why due date changes happen, and what you can learn from them when they do.

Estimated Due Dates 101

Setting an EDD and your baby’s gestational age is one of the main purposes of your first-trimester ultrasound. This is done by measuring your baby’s CRL, or crown-rump length, which can show with fair accuracy how long a baby has been growing in utero.

In a normal, healthy pregnancy, EDD is usually cemented during your second ultrasound. The second ultrasound includes a fetal anatomy scan that measures other parts of your baby’s body to ensure it’s developing at a normal rate. However, a change in due date may be suggested if your ultrasonographer detects a discrepancy between the provided date of your last menstrual period (LMP) and your baby’s growth rate.

What is Naegle’s Rule?

Many providers use a principle known as Naegele’s Rule to calculate the estimated due date. Naegle’s rule hinges on two assumptions:

1. A woman’s pregnancy should last approximately 280 days (40 weeks)
2. A woman’s pregnancy should be dated from a woman’s last period

Franz Karl Naegele was an obstetrician who died in 1851, and his rule has been used for well over 150 years. Physicians who use it apply a simple formula to arrive at an EDD:

• Pinpoint the first day of your last period
• Count three months from that date
• Add 1 year and 7 days to that date

Naturally, the rule doesn’t account for the many anomalies introduced by the realities of women and their reproductive health. From irregular cycle lengths to simply not remembering the start date of your last menstrual cycle, exclusively relying on Naegle’s rule can’t reliably predict a baby’s due date.

Imaging technology like ultrasounds—which should be administered at least twice during a pregnancy—help to account for this and clarify an estimated date. Other than an ultrasound measurement, some providers also use other methods to arrive at an estimate, like measuring the size of your uterus or using another calculation known as Parikh’s formula.

5 Reasons Why Your OBGYN Could Change Your Due Date

In most cases, the ACOG discourages changes to EDD. They elaborate that changes “should be reserved for rare circumstances” and “documented clearly in the medical record,” as they can significantly alter prenatal care plans.

One of the main reasons why your EDD might change is simply a lapse in reliable memory. According to research, only around 50% of women can successfully remember the date of their last period.

Apart from a misrecollection of that important data point, your due date could be subject to change for any of the following 5 reasons.

#1 You’re Pregnant with Twins

One of the most common occasions an EDD might change is when your healthcare provider discovers you’re pregnant with twins or multiples. Carrying more than one baby can make pregnancy—and delivery—more complex because of:

• Different rates of development – It’s common for babies who share a uterus to develop at different rates after the 26th week. In fact, identical twins who share a placenta are at a high risk of a condition called selective IUGR, wherein one twin grows steadily while the other doesn’t grow as well.
• Third-trimester delays – Most twin and multiple pregnancies stall in the third trimester, around the 26-week mark, when the tight space compared to fetal size tends to restrict growth. Though this can create a much more sensitive delivery process, it’s largely considered normal in the world of obstetrics.
• Increased importance of EDD accuracy – The more babies are added to a pregnancy, the more important it becomes to closely observe fetal and maternal health. Miscalculations in EDD as well as other important vitals, may misguide medical decisions if you or your little ones need more care.

The majority of twin or multiple pregnancies arrive earlier than singletons, which must arrive before 39 weeks to be considered premature. In contrast, many twin pregnancies result in premature births. Birth at 37 weeks is considered healthy for twins, as babies usually have sufficient lung capacity to breathe on their own.

If you’re carrying multiples, it’s recommended you be prepared for labor by at least 28 weeks into your pregnancy (and be mentally prepared for your EDD to switch).

#2 Your Pregnancy History

If you’ve been pregnant before, your OBGYN will take your pregnancy history to look for trends that might impact your current one.

Mothers who’ve delivered late or early in previous births are more likely to repeat that pattern. If you have a history of preterm or post-term birth, your care provider may recommend more thorough or frequent prenatal screenings to more accurately predict your baby’s due date.

#3 You Get an Infection

Getting sick during pregnancy could delay your baby’s predicted due date. Some types of infections are very common during pregnancy, including:

• Urinary tract infections (UTIs) – UTIs and recurrent UTIs are common in pregnancy. Even still, it’s important to treat them early with antibiotics. Untreated infections can worsen and cause major disruptions to prenatal care, putting you and your baby at risk of preterm birth or even miscarriage.
• Bacterial vaginosis (BV) – An estimated 25% of expecting moms develop BV while they’re pregnant. BV is the overgrowth of a naturally-occurring bacteria in the vaginal ecosystem. Though BV is common, it can augment your risk of preterm birth and raise your baby’s likelihood of having a low birth weight.

#4 You Have Structural Reproductive Abnormalities

If your provider finds an anomaly in your reproductive anatomy, it can impact your baby’s rate of development. Some conditions that commonly impact EDD include:

• Uterine fibroids – Uterine fibroids are a common type of benign tumor estimated to impact between 20 and 80% of reproductive-aged women. During pregnancy, they can raise the risk of complications like:
  • Preterm birth
  • Cesarean birth
  • Breech position of the fetus

Uterine fibroids can also cause pain, which raises the risk of uterine contractions that could interfere with your original EDD.

• Cervical insufficiency – Cervical insufficiency, sometimes called incompetent cervix, is a clinical term for having an especially weak cervix. Women with this condition are at risk of having their cervix open too early, heightening the chances that a baby will arrive before their original EDD.

#5 You Use Recreational Substances During Pregnancy

It’s universally accepted that using recreational drugs—whether smoking tobacco, drinking alcohol, or other substances—is harmful to fetal and maternal health. However, some expecting moms still use them, particularly if they struggle with dependency or addiction.

The CDC reports the following recreational substances can directly jeopardize pregnancy timing, as well as fetal and maternal health:

• Tobacco
• Opioids

Though marijuana is not directly associated with preterm birth, women who use it are considered at higher risk of untimely delivery. Similarly, alcohol does not cause preterm or spontaneous birth, but it is connected to significant developmental disruptions known as fetal alcohol spectrum disorders (FASDs).

How to Reduce Your Risk of Due Date Changes During Pregnancy

If your OBGYN requests an EDD change, you’ll want to work closely with them to refine your prenatal care and birth plans to match your new date. But, even before a due date change happens, there are some habits you can put into practice to reduce your risk of a schedule change:

• Decide on a concrete prenatal care schedule – Be sure to stick to your prenatal care appointments. If the date of your last period could be incorrect, consider adding more appointments (especially if your provider recommends it).
• Level up your well-being – Maternal health and fetal health are one in the same during pregnancy. From quitting cocktails to boosting your intake of critical nutrients like folic acid, one of the best ways to support your baby is to support you.
• Stay informed about your pregnancy – Try keeping in close communication with your healthcare provider throughout your pregnancy, particularly if you have a significant health history or a pattern of early or late births.

It can feel disconcerting to deal with a due date change—particularly if you’re carrying a high-risk pregnancy, like twins—but the more conscious you can be about your pregnancy moving forward, the better.

Prepare for Your Prenatal Journey with SneakPeek

Every pregnancy has its share of hiccups, and looking ahead to your life as a parent can help you navigate any twists and turns along the way. Even if you’re just beginning your journey, you can learn your future baby’s gender with SneakPeek.

Did you know that you can take a DNA test while pregnant? The SneakPeek Test reveals your little one’s gender as early as 6 weeks into your journey, with over 99% clinically proven accuracy and DNA-based results. You’ll take your gender blood test at home, mail it to SneakPeek labs, and we’ll triple-review your results and return them on the same day they’re received. We’ll even include a complimentary downloadable report you can share with your provider to review.

Join the more than 1 million moms starting parenthood confidently by visiting SneakPeek today.

This post has been reviewed for accuracy by:

Haley Milot

Haley Milot, a highly skilled laboratory professional, currently serves as the Associate Director of Lab Operations at Gateway Genomics, the parent company of SneakPeek. Her extensive experience in laboratory management, spanning over seven years with Gateway Genomics, showcases her expertise in quality assurance, quality control, and the development of laboratory procedures. Haley's background includes pivotal roles in DNA extraction and purification, real-time qPCR, and specimen accessioning, underscoring her comprehensive understanding of laboratory operations and diagnostics. Her adeptness in managing complex laboratory functions and her deep knowledge of medical diagnostics make her a vital contributor to SneakPeek's innovative approach to prenatal testing.

Sources:

1. Verywell Family. When Your Doctor Changes Your Pregnancy Due Date. https://www.verywellfamily.com/questions-to-ask-about-changing-your-due-date-2758648
2. Cleveland Clinic. Ultrasound in Pregnancy. https://my.clevelandclinic.org/health/diagnostics/9704-ultrasound-in-pregnancy
3. The American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
4. National Library of Medicine. Pregnancy Dating. https://www.ncbi.nlm.nih.gov/books/NBK442018/
5. Johns Hopkins Medicine. Calculating a Due Date. https://www.hopkinsmedicine.org/health/wellness-and-prevention/calculating-a-due-date
6. National Library of Medicine. Estimated Date of Delivery. https://www.ncbi.nlm.nih.gov/books/NBK536986/
7. Parents. Pregnancy Ultrasounds Week by Week. https://www.parents.com/pregnancy/stages/ultrasound/ultrasound-a-trimester-by-trimester-guide/#toc-why-do-people-get-pregnancy-ultrasounds
8. Cleveland Clinic. Expecting Twins or Triplets. https://my.clevelandclinic.org/health/articles/9710-expecting-twins-or-triplets
9. National Library of Medicine. Differences in fetal growth patterns between twins and singletons. https://pubmed.ncbi.nlm.nih.gov/30501543/
10. Parents. Twin Ultrasound Images From Every Trimester. https://www.parents.com/pregnancy/stages/fetal-development/twins-a-closer-look-at-your-developing-babies/
11. Mayo Clinic. Twin pregnancy: What twins or multiples mean for mom. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/twin-pregnancy/art-20048161
12. What to Expect. When Will Your Twin Babies Arrive? Your Tentative Timetable With Twins. https://www.whattoexpect.com/pregnancy/twins-and-multiples/giving-birth/your-tentative-timetable.aspx
13. Verywell Family. Typical Gestation Times for a Twin Pregnancy. https://www.verywellfamily.com/are-twins-born-earlier-than-single-infants-2447481
14. Centers for Disease Control and Prevention. Preterm Birth. https://www.cdc.gov/reproductivehealth/maternalinfanthealth/pretermbirth.htm
15. National Library of Medicine. Urinary Tract Infection in Pregnancy. https://www.ncbi.nlm.nih.gov/books/NBK537047/
16. UT Southwestern Medical Center. UTIs during pregnancy are common and treatable. https://utswmed.org/medblog/utis-during-pregnancy/
17. Verywell Health. Bacterial Vaginosis and Pregnancy. https://www.verywellhealth.com/bacterial-vaginosis-in-pregnancy-5272213
18. Healthline. How Do Fibroids Affect Pregnancy and Fertility? https://www.healthline.com/health/womens-health/fibroids-pregnancy
19. Mayo Clinic. Incompetent cervix. https://www.mayoclinic.org/diseases-conditions/incompetent-cervix/symptoms-causes/syc-20373836
20. Centers for Disease Control and Prevention. Substance Abuse During Pregnancy. https://www.cdc.gov/reproductivehealth/maternalinfanthealth/substance-abuse/substance-abuse-during-pregnancy.htm
21. The American College of Obstetricians and Gynecologists. Marijuana Use During Pregnancy and Lactation. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/10/marijuana-use-during-pregnancy-and-lactation?utm_source=redirect&utm_medium=web&utm_campaign=otn
22. National Library of Medicine. Alcohol Intake in Early Pregnancy and Spontaneous Preterm Birth: A Cohort Study. https://pubmed.ncbi.nlm.nih.gov/31803953/
23. Centers for Disease Control and Prevention. Alcohol Use in Pregnancy. https://www.cdc.gov/ncbddd/fasd/alcohol-use.html