Fertility Awareness is awesome! Have I mentioned that before? Not just because it’s an effective hormone-free method of birth control, and not just because it can help you to optimize your chances of getting pregnant, but because there are so many side benefits to charting your menstrual cycles that you probably didn’t even consider when you first started using the method.
For example, using the Fertility Awareness Method gives you the information you need to accurately calculate your due date if and when you get pregnant.
When you are tracking your fertile signs with fertility awareness the most definitive way to identify the date of ovulation is by taking your basal body temperature every morning. Your basal body temperature is a measurement of your metabolic rate. After you ovulate your ovaries start producing progesterone, and one of the things that progesterone does in the body is to increase your metabolic rate which causes a measurable increase in your waking body temperature. You can think of your body as a pretty wicked incubator that heats up after ovulation just in case an embryo implants and needs to be kept cozy and warm. Kind of like when hens sit on their eggs to keep them warm before they hatch.
The fantastic thing about this natural bodily process is that you can easily measure this shift in temperature by getting into the habit of taking and recording your temperatue each morning before you get out of bed. This measurement does not allow you to predict ovulation because your temperature will only increase after you have already ovulated. With that being said, once you have confirmed your temperature shift you know that you have ovulated, and you also know that you ovulated on the day prior to the shift. This information allows you to accurately determine your baby’s due date based on the exact date you ovulated! No guessing!
How is my due date calculated anyways?
Before I got pregnant the first time I had no idea how due dates were calculated. I didn’t know that you are technically “2 weeks pregnant” by the time you ovulate because the first week of pregnancy is actually calculated based on the first day of your last menstrual cycle. By the time you pee on a stick and see that little plus sign you’re already at least 4 weeks pregnant based on the way that due dates are calculated. This is because the soonest that pregnancy tests work is about 2 weeks after ovulation. Right around the time your period was supposed to come, which is technically about 4 weeks after the first date of your last period….technically.
To be specific, a woman’s due date is determined by using a calculation called Naegele’s Rule. Babies have a gestational period of about 280 days (40 weeks), so your doctor will add 280 days to the first date of your last period and voila you have your due date. 
Sounds reasonable. Except that this “calculation” is always based on the assumption that your cycle is 28 days. Not 29 or 21 or 35 but 28 days EXACTLY. The calculation also assumes that you ovulated on day 14 of your cycle.
Here’s what happened when I tried to tell my doctor when I ovulated
The first time I got pregnant was about 2-3 months after my husband and I started trying. Since I was charting my cycles I knew that my post-ovulatory phase (or luteal phase) is usually 12 days long, so I knew that I was pregnant by the time I saw 14 high temperatures above the cover-line. The thing is that I didn’t actually ovulate until day 22 of my cycle; one full week after I was “supposed to” ovulate. According to the pregnancy wheel, ovulation happens on day 14 so my cycle didn’t fit neatly into my doctor’s due date calculation.
I waited until three weeks after ovulation to officially pee on a stick and the test was positive. Yay! My husband and I were so excited! But the very next day I started bleeding. I miscarried at a very early stage, 5 weeks to be exact. I can’t help but wonder if I would even have known that I was having a miscarriage if I hadn’t been charting my cycles. Maybe I would have just thought it was my period. Although it was the most painful and heaviest “period” that I have ever had, so that might have clued me in.
When I went to the doctor and told him that I was miscarrying he asked me when the first day of my last period was so that he could calculate how far along I was. I told him the first date of my last period, and I tried to explain to him that I ovulated on day 22 so I was actually 5 weeks pregnant instead of 6 when I miscarried, but I may as well have been speaking Hebrew or something. It went in one ear and out the other. Based on the first date of my last period I was 6 weeks pregnant. End of story.
He sent me for an ultrasound, and even commented on how small the baby was for the number of weeks along I was, because according to his pregnancy calculator I was 6 weeks along. Even though I had been 5 weeks pregnant and I kept trying to explain it to him based on when I ovulated, he just wasn’t processing the words that were coming out of my mouth.
My doctor’s pregnancy wheel calculation chart had the final say as to when my due date was. As far as he was concerned it was 100% accurate. Apparently in medical school he was taught that women’s menstrual cycles are always the same; always 28 days. Ovulation always happens on day 14, and that is really all there is to it.
I mean, every woman ovulates on day 14 every single month right? There is never any variation from woman to woman or from cycle to cycle. So I am left to wonder exactly how accurate this whole due date calculation thing is in general. An awful lot of women get induced every single day. It’s conceivable that most of the time the actual due date is off by at least a few days if not a week or more depending on the situation.
Here’s how I used Fertility Awareness to get the right due date
This experience taught me that I had to learn to speak my doctor’s language. No one is interested in having me explain the day that I actually ovulated. Clearly. So when I fell pregnant the second time I was prepared.
I had ovulated on day 21 of my cycle. So I took it upon myself to calculate 14 days prior to my ovulation date, and then every time I was asked when the first day of my last menstrual cycle was I lied and told my doctor (and my midwife) that it was 14 days before I ovulated. The end result was an accurate due date based on my exact date of ovulation.
I figured that it was a win-win situation. I would get the proper due date assigned to me without having to have any awkward conversations ending in me not being listened to, and my doctors assigning me a due date that was 1 full week earlier than my actual due date. I also avoided having to listen to my doctor tell me that my baby seemed “small” for being 15 weeks along, when I was actually 14 weeks along.
About due dates
In this day and age it is becoming more and more evident that you have to be your own advocate when it comes to your health and your life. For this reason I feel that any practice that gives you more information about your health is powerful and should be utilized when possible. It is an amazing feeling to take away the mystery of conception and know the exact date of ovulation, and then use this information to accurately pin point your due date.
Pregnancy related care is in many ways determined by the due date and so accurately predicting the due date is important. But with all of that being said, it’s safe to say that there are times when too much emphasis is put on a specific date. Less than 10% of women actually deliver on their due date, and 80% of women deliver at some point between 37 weeks and 42 weeks.   By focusing on being able to pinpoint your due date accurately, by no means am I saying that by doing so your baby will be born on that date. For me it boils down to having accurate information from which to make decisions about my prenatal care. That gives me the confidence to make important decisions and it also gives me peace of mind.
For example, one of my main concerns about the standard due date calculation is regarding the fluctuations that most women have in their cycles. Given that every woman’s due date is calculated the same way regardless of the complexities of her unique cycle, it seems clear to me that the accuracy of this calculation “to the date” needs to be questioned. Keeping this in mind, a woman may consider herself to be 42 weeks pregnant based on her due date calculation, but only be 40 or 41 weeks pregnant, or on the other side may think she’s 40 weeks pregnant when she is really 41. As we all know, serious medical decisions are made based on these dates. And the dates are often treated as gospel, so if they aren’t accurate then the corresponding medical care may be off as well. Inductions happening too early, or too late for example.
Now for the ultrasound
According to this study the variability in a woman’s menstrual cycle can cause that exact issue when it comes to calculating the due date. The study suggests that ultrasound dating is the most accurate way of determining the due date if done in the first trimester when most babies grow at a similar pace. After the first trimester babies grow at increasingly different rates, and the accuracy of ultrasound dating decreases in the 2nd and 3rd trimesters as a result. The results of the study show that many post-term inductions are not actually “post-term” when ultrasound dates are used instead of the standard due date calculations.
Most pregnancies undergoing post-term induction are not post-term when assessed by ultrasound dates. Regardless of whether prolonged pregnancy is considered to be a risk factor requiring intervention, the proportion of pregnancies considered ‘post-term’ can be reduced considerably by a dating policy which ignores menstrual dates and establishes the expected delivery date on the basis of ultrasound dates alone. 
With that in mind, there are significant health implications of having women carry babies “post-term” (beyond the 42 week mark) as stated here
Postdates pregnancies are associated with an increased risk of intrapartum and postpartum obstetric complications and higher perinatal morbidity and mortality rates. Moreover, compared with babies delivered at term, babies delivered postdates are more likely to be hospitalised during the first 3 years of life and are at greater risk of developing conditions including epilepsy, neurodevelopmental deviation and Asperger’s syndrome in later life. 
Given the serious health implications of carrying a baby post-term, having the ability to accurately identify one’s due date can only improve the quality of the prenatal care you receive. It also helps to increase your level of confidence and understanding when it comes to making decisions about interventions that may or may not be required depending on the situation.
Again it turns out that Fertility Awareness is more than just birth control, there are so many unexpected health-related benefits from gathering information about your menstrual cycles. I would argue that women who use fertility awareness don’t need to rely on ultrasound dating because you can’t really get more accurate than being able to pinpoint your exact date of ovulation.
Have you had any children? If so, did you deliver on your due date? If not, when did you deliver? I’d love to hear from you! Please join in on the conversation in the comments below!