The menstrual cycle consists of two phases: the follicular phase (first phase) and the luteal phase (second phase). During each phase, the body goes through a series of hormonal changes and biological triggers that transition it between the phases.
The menstrual cycle follows a cyclical pattern: once you reach the end of a cycle, a new one starts. The first day of period bleeding marks the start of each new cycle. On average, our users’ menstrual cycles last for 29 days; however, the cycle length is very individual and may also vary from one cycle to the next.
Continue learning about the menstrual cycle here.
The follicular phase: the first phase of the menstrual cycle
When does the follicular phase start and end?
The follicular phase begins on the first day of your period. Your period (menstruation) happens when the released egg from the previous cycle doesn’t become fertilized (meaning a pregnancy didn’t occur), so the lining of the uterus which is needed to support a pregnancy is shed in the form of a bleed.
After the end of your period, your body begins to prepare for ovulation. The follicular phase ends when ovulation occurs. Based on the data we've collected from thousands of cycles, the average ovulation day for our Cyclers is CD18.
What hormonal changes happen in the body?
In the follicular phase, estrogen levels are high, and there's an increase in luteinizing hormone (LH) right before ovulation.
What temperatures are expected during the follicular phase?
Leading up to ovulation, the body temperature remains low when compared to the second phase of the menstrual cycle. Once ovulation occurs, your body temperature rises, indicating the start of the second phase of the menstrual cycle (the luteal phase). The average follicular phase temperature for our Cyclers is 36.23°C (97.21°F).
The luteal phase: the second phase of the menstrual cycle
When does the luteal phase start and end?
The luteal phase begins after ovulation and lasts until the next period starts.
What hormonal changes happen in the body?
In this phase, the progesterone levels in the body increase.
What temperatures are expected during the luteal phase?
The rise in progesterone levels causes the body temperature to rise and remain high until the next cycle begins with your next period. The average luteal phase temperature for our Cyclers is 36.58°C (97.84°F).
However, if pregnancy occurs because the egg that was released during ovulation becomes fertilized, the body temperature will remain high and you will not get a period until after your pregnancy.
How can Natural Cycles help me understand my menstrual cycle better?
Natural Cycles is an app powered by an algorithm that learns the pattern of your unique cycle to work out your daily fertility status based on body temperature. You can use Natural Cycles to prevent pregnancy with NC° Birth Control, plan a pregnancy with NC° Plan Pregnancy, or follow your pregnancy with NC° Follow Pregnancy. Just measure with your NC° Thermometer, or wear the Oura Ring or Apple Watch overnight, and let our algorithm do the work.
How can an app be a reliable birth control method?
The body temperature changes during the menstrual cycle as a result of hormonal changes — your body temperature increases after ovulation due to the rise in progesterone — so measuring your body temperature is an indicator of whether or not ovulation has occurred. These temperature changes are small (but significant) so the measurements must have two decimal points.
The Natural Cycles algorithm analyzes your temperature and period data to find ovulation, and you can give the algorithm an extra boost by taking optional ovulation tests. The algorithm also takes into account other factors like sperm survival, temperature fluctuations, and cycle irregularities to identify ovulation and calculate your Red Days (fertile) and Green Days (not fertile).
Natural Cycles is the first birth control app to be cleared by the FDA in the United States, and we're CE-marked in Europe. Our certifications have been granted based on mandatory clinical data that demonstrates that the effectiveness rate of Natural Cycles is 93% with typical use and 98% with perfect use (when using protection on each Red Day). Most other fertility apps assume your cycle is 28 days long and that you always ovulate on Cycle Day 14. This means the fertile window they give you is not tailored to you. Our algorithm significantly reduces the risk of you being assigned an incorrect fertile day as it uses your personal data. This reduces the risk of unwanted pregnancy, or can help you plan pregnancy with greater precision. You can read more about Natural Cycles’ effectiveness here.
Natural Cycles also helps you gain insight into your unique cycle, so you can get an overview of the most important information to help you manage your health. This is shown in the Cycle Insights page in the app (accessible from the Today view > See cycle insights), which is updated frequently as you add more data.
Natural Cycles can be your long-term fertility partner, and you can seamlessly switch between NC° Birth Control, NC° Plan Pregnancy, and NC° Follow Pregnancy depending on your journey.
While some fertility apps make money by selling data for advertising, we never have — and never will — sell our users’ data. Read more about how we protect your data here.