The follicular phase is the first phase of your cycle. It starts with your period and ends with ovulation. During this phase, your body is preparing for ovulation. This means there’s an increase in hormones that initiate ovulation: luteinizing hormone and estrogen.
The luteinizing hormone levels (LH) rise roughly 48 hours before ovulation, so an increase in LH levels can indicate that you may ovulate within the next 48 hours. An increased LH is a strong indication that ovulation may soon occur, but not a confirmation that ovulation has or will occur. During the follicular phase, the Natural Cycles app may prompt you to take LH tests (also known as ovulation tests) which measure levels of the luteinizing hormone in the urine. These tests are optional and not required by the algorithm, but when used, they are taken into account by the algorithm to help narrow down your fertile window. You can find out more about LH tests here.
Towards the later stage of the follicular phase, levels of the hormone estrogen increase. This causes the lining of the uterus to thicken in preparation for the implantation of a fertilized egg cell. At the same time, the rise in estrogen also causes an increase in cervical mucus, which promotes sperm survival and makes it easier for the sperm to travel up the fallopian tubes.
How long is the follicular phase?
Since all our cycles are unique, the length of the follicular phase varies from person to person. Our study of 600,000 cycles found that, typically, the follicular phase is the longest phase of the menstrual cycle and is, on average, 17 days long.
While all of us will experience some irregularities from time to time, those with irregular cycles can expect to see a variation in both the follicular phase and the luteal phase from cycle to cycle. The follicular phase is however, the most variable phase for everyone.
If you’re curious about your own cycle phases, you can see this individualized data on the Cycle Insights page, which you can access via the Today view > See cycle insights. There, you can compare your cycles or take a look at the average length and variation of your own cycle phases.