The cycle begins with menstruation, the phase when the uterus sheds its lining.
FSH tells the ovaries to make immature eggs, calledfollicles.
These follicles secrete the hormoneestrogen, which has effects throughout the body.
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During ovulation the mature egg is released from the most dominant follicle and travels through thefallopian tubesto the uterus.
The leftover follicle becomes a hormone-secreting cluster of cells called thecorpus luteum, meaning yellow body in Latin.
The corpus luteum secretes several hormones, includingprogesterone.
Illustration by Katie Kerpel / Verywell
Progesterone causes the uterine lining to thicken in preparation for accepting a fertilized egg.
Progesterone also causes changes in cervical secretions that promote sperm motility.
The ongoing progesterone secretion maintains the uterine lining for pregnancy.
The Fertile Window
The fertile window is the period in which sexual intercourse can result in pregnancy.
After ovulation, the egg can live for up to 24 hours.
Sperm can live in thefemale reproductive tractfor up to five days.
Illustration by Katie Kerpel / Verywell
What Is Basal Body Temperature?
BBT is the bodys temperature when in a state of rest.
During the first half of the menstrual cycle, from menstruation until ovulation, the temperature is relatively low.
Once ovulation occurs and progesterone levels rise, the body temperature increases by up to 0.5 degrees.
The temperature remains relatively high until progesterone levels drop and menses occurs.
This information can be useful for people trying to conceive and for those trying to avoid pregnancy.
People trying to conceive can use BBT topredict ovulationin order to time intercourse during the fertility window.
Anything less accurate can miss the small changes needed for fertility prediction.
Some thermometers are designed specifically for BBT that may provide some added value.
When measuring BBT, the body should be completely at rest.
Use this information to predict the timing of ovulation and your fertile window in future months.
Hormones cause several other changes in the body.
A couple of findings on the BBT chart can be early signs of pregnancy.
Of course, suspected pregnancy should be confirmed with a pregnancy test and a visit to your healthcare provider.
One very early potential sign of pregnancy is a one-day drop in temperature, called the implantation dip.
An implantation dip is not the most reliable sign of pregnancy because it may not be present.
If it’s present, it does not always mean pregnancy.
The dip can occur about a week after ovulation if fertilization has occurred and lasts for just one day.
A more predictable and noticeable change in BBT that can indicate pregnancy is a prolonged increase in temperature.
During pregnancy, progesterone levels remain high and cause a sustained increase in BBT.
Look for otherpregnancy signs, like a missed period and symptoms like fatigue and nausea.
BBT relies on charting daily temperature and noticing small changes over time to predict future fertility windows.
A Word From Verywell
Trying to get pregnant or avoid pregnancy using hormone-free methods can be challenging.
Not everyone is able to reliably predict ovulation, particularly those who have irregular menstrual cycles.
Sperm can survive for up to five days.
A released egg can survive for up to 24 hours after ovulation.
Adding these together give you a fertile window of approximately six days.
It will coincide with a missed period (or very light spotting from implantation bleeding).
Always confirm suspected pregnancy with a pregnancy test.
Simmons RG, Jennings V.Fertility awareness-based methods of family planning.Best Pract Res Clin Obstet Gynaecol.
2017;2(3):238-246. doi:10.1002/btm2.10058
Fertility Friend.Implantation dip study.
