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 implant 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 temperature 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 “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 to 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!
I haven’t delivered. In fact, I’m only 4 weeks along today (or 5 weeks if you use standard calculation), and I’ve just been reading around trying to find out how to get my actual due date.
This post is awesome and very helpful. Luckily I also knew which day I ovulated and tacked 14 days onto it.
It is very important to be able to calculate accurately. I have a sister that goes overdue with every baby without fail, and I’ve always wondered why.
It’s so odd to think that doctors in this day and age can’t fathom that every woman is different.
Luckily, I have a female doctor who seems to understand better that one size doesn’t fit all.
Great post. 🙂
Congratulations!! Wishing you a safe and healthy pregnancy! I couldn’t agree more Marie! Doctors seem to be taught that all women have 28 day cycles without fail, even though that is clearly not the case.
Thank you so much for this post! I got pregnant on my first cycle of charting, fresh off of the birth control pill… so I didn’t ovulate until day 31 of my cycle! I have been so confused about due dates until reading this. Now I know that I am 3 weeks and 5 days pregnant, not 5 weeks pregnant like the doctors keep trying to tell me. It is so confusing when they disregard our charts! Thanks again!
Hi! Loved your website. I have 2 questions:
I don’t know if I understood it well. So the first day of pregnancy is the first day of the cycle which you got pregnant? (many days BEFORE the person ACTUALLY got pregnant????). This doesn’t seems reasonable at all. If the gestational period is 280 days, that means 40 weeks starting on the day of CONCEPTION, not the first day of the period. Even for someone with 28 day cycles: when she gets to the “40th week” of pregnancy (considering this weird formula they use), her baby has been developing for 38 weeks, not 40.
If you took the test 3 weeks after ovulation, this means that, that day, you were 3 week pregnant right? Why do you say you were 5 weeks? After wall, the conception happened around the day you ovulated, so you where 3 weeks pregnant.
Thank you! So glad you like the site. Yes you are right. Technically the first day of your pregnancy is the first day of your menstrual cycle (i.e. the first day of your period the cycle you conceive). And this is based on the 28 day textbook cycle where you are considered to have ovulated on day 14. So if you were to test for pregnancy 3 weeks after ovulation you would be 5 weeks pregnant although you only conceived 3 weeks ago.
Hi! I really enjoyed this read, thank you! I very much agree!
I cant figure out my due date, (i dont think the doctor is right) do you think you could help?
If you have been charting your cycles and you can identify your date of ovulation with your temperature shift, then you can use any online due date calculator. Just put in the “first date of your last menstrual period” as two weeks prior to your date of ovulation and you will have a more accurate due date. If you don’t know when you ovulated then you would be best to have your doctor use ultrasound dating because that would potentially be more accurate than trying to guess.
Hi, this is so interesting. I have a feeling that my date is off, because I feel much smaller than most although everyone says everyone is different which I also believe but there were lots of scenarios that made me doubt. Like I didn’t officially know until I was 4 months and I started growing much later etc. My cycle is usually every 6-8 weeks, and I still bled twice during my first trimester, although the last cycle was spotting. Can you help me establish roughly my due due, as i’m scared i’m way off and I’m seriously considering asking not to be induced and rather wait for it to come naturally… my last real period was 13th Friday March and slight spotting for a few days on Sunday 26th April. Although I did a pregnancy test a few days before 13th Feb coz I was so late and my breasts felt really tender, but it came out as negative… I would love to hear from you with some advice. I’m planning a home birth and as little medical interaction as possible so would love to know about the due date accuracy as this will make a huge difference.
In your case it would be hard for me to help you figure out your due date unless you were charting your cycles. If you are tracking your cervical mucus or taking your morning basal body temperature each day then it is pretty easy to figure out when you ovulated and use that date to calculate your due date. But if you had mid cycle bleeding it is hard to look back and figure out which bleed was actually menstruation and which bleed was mid cycle spotting. Feel free to send me an email if you want to delve into it further: info [at] fertilityfriday [dot] com
This article is spot on and I know from experience. I will try to keep it as brief as possible. Basically, when I fell pregnant with my son, I had a couple of bleeds within a month so was unsure of when my last period was. However, the midwives / doctors never listened to me and went by the earliest bleed. I was baffled when my due date was mid October instead of mid November. But when it is your first pregnancy, you trust the doctors. At my scans, they put the due date further and further back to end of October. However, they didn’t want to put it too far because of my LMP. Even though I told them I was unsure of the LMP, they just ignored me. Speed forward to end of October, I had no signs of delivering soon, I felt great, wasn’t particularly big or uncomfortable. When it got to 6th November they told me I had to be induced. I was petrified because of the unknown. So, went along with it, even though my instincts told me no and I felt bullied! It took 3 days of induction and epidural to get him out. My placenta didn’t deliver properly and had to be pulled out manually (yuck!) And my bladder packed up because of the epidural. My son was not a large baby and was distressed to say the least. I was, and still am gutted about it all. He should have been born later. He was forced out and I feel so guilty about that. I have just found out that I am pregnant with number 2 and there is no way I am letting that happen again. The doctors and midwives couldn’t even give me a straight answer when I asked them why I was being induced. I will not be bullied this time. I know my due date and even if my child is 2 weeks late I am not being induced. It is not worth it.
I am so sorry to hear about that. Your gut instincts were right on, but doctor’s don’t seem to stray much from that LMP date even if you try to provide them with an explanation. Wishing you the very best with your second pregnancy! And wishing you a safe delivery when the time comes 🙂
I’ve had two boys, both were born at 38 1/2 weeks exactly. Both due dates were confirmed based on my charts using fertility awareness. I think what you are saying is very important but I’ve also come to wonder if since, each person is different, perhaps each person in the womb is different, too. Like, maybe one baby needs to be in the womb for 39 weeks, but then another baby needs the full 40. Each child progresses at a slightly different pace after they are born, perhaps it’s similar before? I dunno, just thinking out loud. Love your site!
That is interesting Ashley. I’ve wondered the same thing. Both of my boys were born at 38 weeks. Perhaps there are differences between women in terms of the length of gestation…
Was just googling to see if other women have had children born on their accurately predicted due dates based on fertility awareness. I have 2 children born precisely based on my calculations (well the 1st was 30 min late – born at 12:30am the following day). But I didn’t realize until after the fact that they both born exactly on time, not 6 and 11 days late respectively. I am about to deliver #3 and had I told the medical staff when I had my last period this baby’s due date would have been 12 days before the one I calculated. And my 20 week ultrasound said the baby’s due date was 5 days before the one I calculated. So here I am with just 4 days (counting the due date) til MY calculated due date. So I really think I may have nailed it once again! I am just curious if this happens to other women because it is REALLY awesome to be able to do this.
I’m really confused about being 2 weeks pregnant before you’re actually pregnant- this doesn’t make any sense to me. Why do doctors add 2 weeks to the 280 days?? If you know when you ovulated why wouldn’t you add 280 days to that date to get your best due date?
Hi, this is so interesting. I haven’t delivered and I’ve just been reading around trying to find out how to get my actual due date. I read your article & tired “Due Date Calculator” on whattoexpect. The post is helpful. With information from your post I could easily track on my due date also I knew which day I ovulated. It is very important to be able to calculate accurately.
This is super amazing to me! I have my first Daughter late they induced me yet my body was going into labor naturally.. she was due the 8th had her the 11th. Now im pregnant and was given two different due dates.. One was for the 2oth of April and then went in to see a new Dr and they said the 16th of April.. then I got a ultra sound and the radiologist said April 2nd.. and the last one makes the most sense based on what my Real period on June 22nd.. I had a hematoma that happened in July I bleed for like not even 30mins and they are doing their predictions from that… it just worries me because I don’t want to be over due like you said the complications in having a baby late she measuring RIGHT if I go by my REAL last period not the 30mins bleed.. im terrible at the ovulation thing would you maybe have insight in that? Dr. don’t really listen or help lol lol I just want to make sure I make good informed decisions and shes totally healthy!!
Fertility Friday says
Hi Shannon, it’s harder after the fact to go back and think about when ovulation could possibly have happened, but when you chart your cycles, you’ll have a very good idea (within a day or two) when you pay attention to your mucus patterns and BBT shift!
My son was born 5 weeks ‘preterm’. I have short cycles around 21-23 days. They held us in the NICU for two weeks after he was born. I never felt he needed the extensive medical care. This is hard for me to contemplate now that he’s almost 3. I only came to fertility awareness in the last year. This article was very helpful to me in trusting myself and my intuitions are not crazy. Thank you!