Why are estimated due dates so important?
Besides giving the mother and family a time frame to expect their new addition, estimated due dates allow your care provider to determine your baby's gestational age. Since there are certain developmental milestones and approximate sizes reached each week and month of gestational age, care providers use this dating method to ensure that the baby is getting adequate nutrition and blood-flow from the placenta.
Estimated due dates are also crucial in determining when it is safe to allow a mother's labor process to continue on through delivery. Labors that start before 37 weeks gestation are considered pre term and may pose great health risks or even death to the baby if the labor cannot be stopped. The closer you are to 37 weeks, which is known as early term, the lower these risks become.
Gestational age is also important if the mother or baby develop health issues, like gestational diabetes or intrauterine growth restriction, and an induction or cesarean section before the baby reaches full term becomes a necessity.
Even with normal, healthy pregnancies it is important to know when your baby is due. Many care providers are very comfortable routinely offering inductions at 39 weeks when the baby is considered to be full term, some even insist upon them. The issue then falls on whether or not the baby is in fact truly 39 weeks gestational age when the induction is performed, as the last few weeks in pregnancy are crucial in the baby's lung, digestive tract, and body fat development.
Where can you find out your estimated due date and how are they calculated?
There are hundreds of websites and apps available that offer to help calculate your due date. Some are very simple and show your Estimated Due Date (EDD) calculated by the first day of your last menstrual period (LMP). More complex apps base your EDD off of ovulation date or the date of conception. Some pregnancy calculators may even give you a day-by-day count down to your EDD, including when each trimester starts, and when you can have certain ultrasounds or prenatal testing done.
Your care provider will also calculate your EDD based off these methods, with LMP calculations being the gold standard. Later your EDD can be confirmed when you are between 10 and 14 weeks along by an elective dating ultrasound. During this ultrasound they will measure your baby's crown-rump length (CRL) to determine their gestational age. Since most fetuses grow and develop at a similar rate until the 14th week this measurement is considered to be fairly accurate, and your EDD may change based off the results.
How accurate are these calculations and measurements?
Well, quite a few factors come to play when determining the accuracy of an EDD. The first being whether or not you have a "textbook" 28 day cycle. Most online pregnancy calculators, apps and your care provider use a specific calculation that assumes you have regular cycles 28 days apart. This calculation gives you an EDD that is exactly 40 weeks after your LMP date.
Here is why this may be inaccurate if you have irregular cycles: it takes an average of 14 days after ovulation for an egg to be fertilized, travel through the fallopian tube, turn into a blastocyst (fancy word for a fertilized egg that has yet to become an embryo) and embed into the uterine lining in a process called implantation. Once implanted your body starts producing the hormones necessary to maintain the pregnancy. Based on the 28 day model, this would mean that you would have to ovulate the 14th day of your cycle.
For women who have cycles that are longer or shorter than 28 days this is problematic. For example, if you consistently have a 35 day cycle you likely ovulate closer to the 21st day of your cycle, meaning your EDD would be close to a week off. While that may seem like a small margin of error, that one week could make a big difference for your baby's development, especially if an elective induction or planned cesarean section are a possibility.
Other issues in determining an accurate EDD based off and LMP date involve women who may have fertility or other health issues, such as polycystic ovary syndrome or diabetes. These issues may cause them to have several months in between cycles. It is possible that these women may be several weeks to a few months into their pregnancies before suspecting anything, or they may only be in the very early stages of pregnancy but their last cycle started many months prior.
For women with irregular cycles, using your ovulation date may be a more accurate method to determine your EDD.
While using your ovulation date is generally more accurate than using the your LMP to calculate an EDD, unless you have been religiously tracking your fertility it may be tricky to determine precisely when ovulation occurred during the cycle you concieved. Some women have signals that indicate ovulation has occurred, such as tender breasts or spotting, however these signs usually occur 24 hours or more after ovulation. Other women may experience mittelschmerz (a German word meaning "middle pain"), which is mild to moderate cramping associated with ovulation. This type of cramp usually is a sharp, acute pain that tends to focus on the right or left side and usually differs from menstrual cramping.
It should also be noted that many women do not experience any ovulation symptoms, or are not attuned to them so they are dismissed or ignored. Tracking your fertility signs like your basal body temperature, cervical position, and cervical mucous consistently over two to three months can help pinpoint your ovulation day. Keep in mind many care providers doubt the accuracy or ability of most women to be able to determine what day they ovulated, so typically prefer to base calculations off of LMPs or crown-rump-length measurements.
Another way of calculating your EDD is pinpointing the exact date of conception.
Though this may be tricky or impossible unless you have been scrupulously keeping track of your fertility signs and/or only had intercourse once or twice during your fertile window. You may also know your conception date if you have undergone fertility treatments like IUI or IVF.
The final factor that determines the accuracy of your EDD is your baby.
Spontaneous labor, meaning a labor that has not been induced by any type of intervention, occurs when a number of cues from your baby signal your cervix to start its process of dilation and effacement (thinning). While we have yet to discover what ultimately causes spontaneous labor to start, it is theorized that your baby may produce a hormone that stimulates labor when their lungs are fully mature. Other women go into preterm labor either for undetermined causes or related to health issues with the mother or the baby. Still more women, albeit a smaller percentage, go two weeks or longer past their EDDs without any signs of their body intending to go into labor and have to be induced.
So exactly how likely is it that you will go into labor on your estimated due date?
Only 4% of babies are born on their actual estimated due date.
90% of babies are born within the two weeks prior and the two weeks following their EDD. Studies have shown that most women actually go into labor closer to 41 weeks gestation versus the traditional 40 week model. Because of this, I encourage expecting mothers to try and not to get hung up on a specific date; instead mark off the two weeks before and after your EDD as your "Birth Month" vs a single day circled on your calendar.
Why is this important to keep in mind?
It is normal for a mother to feel emotionally and physically exhausted by the time she reaches her final weeks of pregnancy. Most mothers are extremely uncomfortable and are anxious for labor to start. Many care providers routinely start checking for dilation and effacement at the 37 week prenatal appointment. Mothers, especially first time ones, may feel a deep sense of disappointment or defeated when they are told they are not "making any progress". In situations like these it is important to remember to trust in your baby and your body; your labor will start when your baby is ready to be born. It is worth the wait!
I would love to see expecting mothers post their Birth Months and new mothers willing to share their EDDs along with their little one's actual birthday in the comments below!
Also, check out Evidence Based Birth's article about inductions to start labor in women who have gone past their due dates. It goes into great detail about how EDDs are calculated, their accuracy and provides links to several studies supporting this information.
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