How Often Do Women Ovulate on Day 14? (Episode 521)

Full Transcript: Episode 521

Lisa Hendrickson-Jack:

In today’s episode, I am tackling one of the most common, prevalent, and persistent myths about the menstrual cycle, which is menstrual cycle length — the myth that all menstrual cycles are 28 days and that ovulation occurs always on day 14. And what’s great about this episode is that I have a fantastic study to share with you. This study is one of the first of its kind. With the advent of menstrual cycle apps — in the study they mention that there are over 100 fertility awareness-based mobile apps with over 200 million downloads — when we have this type of data, it really gives us an interesting opportunity to learn a lot more about the menstrual cycle. And so in the study that we’re going to talk about today, they did just that. They gathered data from the many, many users of one of the apps, and they analyzed over 600,000 real-world menstrual cycles. This is a passive analysis of existing data — these women were just users of this app and the researchers were able to extract that data. But either way, it gives us a really interesting insight into a huge amount of menstrual cycles in the real world. And so that is what we’re going to be talking about today — comparing it to the common myth that menstrual cycles are always 28 days and ovulation is always taking place on day 14. So without further ado, let’s go ahead and jump into today’s episode.

You’ll find the link to today’s study over at fertilityfriday.com/521. The study that we’re going to be going over today is called Real-World Menstrual Cycle Characteristics of More Than 600,000 Menstrual Cycles. This study analyzes over 600,000 menstrual cycles, which is really, really interesting. They had, according to the study, over 120,000 anonymized users of the app. These women were residents of Sweden, the UK, and the USA. The average age was about 30 years old, and the range was between age 18 and 45. Eighty percent had started using the app specifically to prevent pregnancy, and 20% had used the app specifically to plan pregnancy. There was somewhere between an average of about eight to nine cycles per user. Out of 1.4 million cycles that were recorded by all eligible users, they actually excluded quite a bit. They kept only cycles between 10 and 90 days, and they excluded cycles where they could not detect and confirm ovulation. That left them with just over 600,000 cycles to include in the study.

One of the stats that really stood out to me: out of all 600,000-plus cycles, only 13% of those cycles were actually 28 days long. There was a separate study that I have quoted before that looked at how many cycles actually had ovulation on day 14 — and it was a similar stat, like 13.5%. So that gives us a sense of what’s really going on here. It’s not to say women never ovulate on day 14 — of course they do — but the vast majority of the time, according to this study, 87% of the time, they don’t. And I think that’s a really key, useful takeaway.

There’s variation — that’s the biggest takeaway. The average follicular phase length was 15.4 days. So on average, women are ovulating on day 15. And the luteal phase — the second half of the cycle — the average was 12.6 days. The average cycle is about 29 days in length, which is exactly in line with what this study found. A normal cycle tends to fall somewhere between 24 and 35 days in length, and in order for a cycle to fall in that range, ovulation generally needs to take place between about days 10 and 23.

The overall average follicular phase length was 16.9 days. When broken down by cycle length, for women with cycles of about 25 to 30 days, the average follicular phase was about 15 days. For women with normal but longer cycles — 31 to 35 days — the average date of ovulation was about 19.5. For women with shorter cycles of 21 to 24 days, the average ovulation took place around day 12. In very short cycles — shorter than 20 days — the average follicular phase length was 10 days, meaning some women are ovulating even earlier than that. And in women with very long cycles of 36 to 50 days, the average follicular phase length was about 26.8 days. So what we have here is a whole lot of fluctuation. There are women who are ovulating before day 10, and there are women who are ovulating after day 30. The length of the follicular phase — the date of ovulation — is the most significant factor in determining whether a cycle will be short or long.

A normal cycle ideally falls between 24 and 35 days, and we are looking for ovulation to take place somewhere between days 10 and 23. But that does not mean ovulation can’t take place outside of that range, particularly in cycles where we might need to be looking a little bit deeper. The average across many studies is quite consistent — average ovulation typically falls around cycle day 15, and overall cycle length typically lands somewhere around 29 days.

One of the things I highlighted from the study was a comment the researchers made about the luteal phase. They said it is remarkable that short cycles had a significantly reduced luteal phase relative to normal-length cycles, but conversely, very long cycles had a significantly long follicular phase, and the luteal phase didn’t vary much. In more approachable language: if the cycle was shorter, the luteal phase tended to be shorter. But when the cycle was longer, the luteal phase didn’t really vary that much from normal cycles. The overall point is that the luteal phase is the most consistent part of the menstrual cycle.

However, the study also highlighted that the myth that the luteal phase is always 14 days is just that — a myth. There is a range. The luteal phase can vary anywhere from 7 to 19 days. From my own experience, I would be more comfortable putting a maximum of 17 days, because you don’t really see luteal phases longer than that on a regular basis. But even at 7 to 17 days, that is a much narrower range than the follicular phase, which can really range up to over 20 to 30 days in cycles outside of the optimal range.

After ovulation, the ovarian follicle transforms into the corpus luteum — also called the yellow body — a temporary endocrine gland that forms on the ovary and releases progesterone for the duration of the luteal phase. If the egg is not fertilized, the corpus luteum has a certain lifespan. If it is fertilized, the fertilized egg starts to produce hCG — human chorionic gonadotropin — which essentially rescues the corpus luteum and prevents its demise, signaling that progesterone production needs to continue. If the corpus luteum is not rescued, it disintegrates. This is a physiological fact. So what determines the length of the luteal phase is the strength of progesterone production — the health of that corpus luteum, its ability to sustain and survive. When certain conditions reduce our ability to produce progesterone, that can shorten the duration of the luteal phase and lessen the actual output of progesterone during that phase.

Another interesting data point: out of the whole dataset, 18% of cycles had a luteal phase of less than 11 days — that is almost one in five cycles. The study also cited another researcher who found that 15% of cycles viewed were less than 11 days, and went on to say that it has been proposed that all luteal phases of less than 10 days and 74% of 10-day luteal phases are abnormal, implying inadequate progesterone secretion. This is just another nod to how the menstrual cycle can be used as a vital sign. We can get this data without a blood draw. It can certainly alert us to the potential need for further testing and can help us identify hormonal issues in a much less invasive way.

The study also looked at age and BMI. As women get older and approach perimenopause — that 10-year window before the last period — cycles tend to start getting shorter because ovulation begins to happen earlier in the cycle. The researchers found that average cycle length dropped by 3.2 days from age 25 to 45, and the average follicular phase dropped by 3.4 days during the same period. It’s really the follicular phase that is determining the shorter cycle length — ovulation is the most variable part of the cycle and varies much more than the luteal phase.

On BMI, the researchers found quite subtle results. They were not able to determine significant effects of BMI on ovulatory menstrual cycle characteristics. The strongest effect they found was a 14% higher rate of cycle length variation in women at the highest BMI levels compared to women with a normal BMI. But overall, there wasn’t much difference in cycle parameters just because of a woman’s weight. This is one of the reasons I love looking at the menstrual cycle as a vital sign — a woman can have a normal menstrual cycle independent of weight differences. Weight in and of itself doesn’t mean someone automatically has PCOS. There are plenty of lean women who suffer from that condition and have that long, irregular cycle presentation.

To conclude: the menstrual cycle is not always 28 days. Women do not always ovulate on day 14. This is not how we should approach cycle charting or any goal-oriented cycle-related activity. If you’re trying to conceive, we really should be looking at your individual cycle — when you start showing cervical mucus, when your ovulation actually is — and we shouldn’t be making assumptions. You should be paying attention to mucus first, because many women ovulate before day 14 and many women ovulate after day 14. And only 13% of those menstrual cycles were 28 days.

I love this study. I think it’s really useful to highlight some of the things I’ve been saying for over two decades. And I love it when we can actually get data — especially in a study with this much of it. Looking at this real-world data is so interesting, so beneficial, so useful, and so helpful — especially for those of you who might have had some pushback if your cycles are not always 28 days and you’ve had difficulty conveying that to a provider, or if you’ve been working with clients who have had some of these challenges and you’ve been trying to educate them on how the menstrual cycle really works. This data helps us to better convey what the menstrual cycle really does look like in the world and in the wild. I hope that you enjoyed today’s episode.