You’ve probably heard that an average menstrual cycle is 28 days. Of course, “average” isn’t the same as healthy or normal. Some women have 27 day cycles, other women have 33 day cycles, and other women have cycles that vary every month. All of these can be perfectly healthy even though they don’t match the clinical average.
That said, the length of your cycle can be an important indicator of your fertility—especially if you know the relative length of the different phases of your cycle. If you notice that the length of your cycle is changing, or seems particularly short or long, it’s a good reason to do some deeper digging about why.
28 Day Cycle
Let’s start by looking at the average cycle, which is 28 days long. In the average cycle, day 1 is the first day of your period. The follicular phase—when your ovaries are getting ready to release an egg—lasts from day 1 until day 14. Ovulation occurs on day 14, and marks the transition to the luteal phase. The luteal phase lasts another 14 days, and when it’s over, the next cycle begins.
But an average-length menstrual cycle can conceal abnormal length follicular and luteal phases. Here’s what those should look like in a healthy cycle:
Follicular phase: 12 – 25 days
Luteal phase: 10 – 16 days (but usually more like 12-14)
Short Follicular Phase
If your follicular phase is less than 10 days long (in other words, if you ovulate on day 10 of your cycle or earlier), it could indicate that you released an immature egg. Immature eggs are either unable to be fertilized, or can be fertilized and have chromosomal issues.
Long Follicular Phase
If your follicular phase is longer than 25 days, it’s a sign that your body is making multiple attempts to ovulate. If your body loses steam before ovulation actually occurs, you may notice multiple patches of fertile cervical mucus about two weeks apart as your estrogen and LH levels rise again.
What is the reason for multiple estrogen/LH surges and no ovulation? This often happens in PCOS. But it can also happen when you’re stressed. Keep in mind that even stress you don’t feel has the potential to impact your cycle. Things like intense exercise, travel, and jet lag all have the potential to delay ovulation and lengthen your follicular phase.
The good news is that a late ovulation does not seem to impact your chances of conception or viability. As long as you eventually ovulate, you have just as good of a chance of having a good egg as you would if you ovulated on time.
Short Luteal Phase
If your luteal phase is less than 10 days long (in other words, you get your period 10 days or fewer after ovulation), it’s a sign that your progesterone levels are too low. Progesterone helps maintain the uterine lining for the egg to implant. A short luteal phase can make it difficult to conceive.
Long Luteal Phase
If your luteal phase is longer than 16 days, you may want to take a pregnancy test. If you’re getting negative pregnancy test results more than 16 days after confirmed ovulation, ask your doctor for a blood test. In rare cases, urine tests will not indicate pregnancy for several weeks.