Fertile Days — How to Determine Ovulation
Fertile days are the period in the menstrual cycle with the highest probability of conception. At Palmotićeva Clinic we precisely determine ovulation through folliculometry and advise couples planning pregnancy or seeking reliable contraception.
What Are Fertile Days?
Fertile days are the period in a woman's menstrual cycle when the probability of conception is at its highest. During the cycle, the ovary releases an egg cell ready for fertilisation — this process is called ovulation. Understanding fertile days helps both couples planning pregnancy and those who want to better understand their reproductive system.
How Does the Menstrual Cycle Work?
The menstrual cycle is the period between two menstrual bleedings. The first day of menstruation is counted as day one of the cycle. The average cycle length is 28 days, but the normal range is 24 to 35 days.
The cycle is divided into two phases:
- Follicular phase (before ovulation) — variable in length, the ovary prepares a follicle containing the egg
- Luteal phase (after ovulation) — almost always lasts 12–16 days (average 14), regardless of overall cycle length
When Are the Fertile Days?
A released egg lives for 12 to 24 hours, while sperm can survive inside the female reproductive tract for up to 5 days (average 3 days). Based on this:
- Most fertile period — 5 days before ovulation and 1–2 days after ovulation
- In practice — from 19 days before the next period to 10 days before the next period
- In an average 28-day cycle, fertile days are approximately from day 9 to day 18
How to Accurately Determine Ovulation
Several methods exist for determining ovulation:
- Calendar method — tracking cycle length over several months, but not fully reliable
- Basal body temperature — a rise of 0.3–0.5°C after ovulation
- Cervical mucus — becomes clear, stretchy and egg-white-like during fertile days
- Ovulation tests (LH tests) — detect a surge in luteinising hormone 24–48 hours before ovulation
- Folliculometry — a series of ultrasound examinations at our clinic that precisely track follicle growth and confirm ovulation
For couples actively planning pregnancy, folliculometry is the most precise method, performed at Palmotićeva Clinic.
Fertile Days and Contraception
Many couples mistakenly believe that the cycle is exactly 28 days and that ovulation occurs on exactly day 14. This approach often leads to unplanned pregnancy. It is important to know:
- Not every bleeding is menstrual bleeding
- The cycle does not last exactly 28 days for most women
- It is very difficult to pinpoint the exact day of ovulation without additional tracking methods
- The calendar method as the sole form of contraception has a high failure rate
If you do not wish to become pregnant, we recommend using reliable contraception and consulting a gynaecologist about the best option for you.
When to Visit a Gynaecologist
We recommend a gynaecological examination in the following situations:
- You have been trying to conceive for more than 6–12 months without success
- You have irregular or missed periods
- You want precise ovulation tracking (folliculometry)
- You need contraception advice
- You experience pain or bleeding between periods
Written by
Dr Slobodanka Petković
Specialist in Gynaecology & Obstetrics · 35+ years of experience
Patients often ask
Fertile days occur around the middle of your cycle — approximately 5 days before ovulation and 1–2 days after. In an average 28-day cycle, this is from day 9 to day 18. For precise determination, we recommend folliculometry at the clinic.
Folliculometry is a series of ultrasound examinations at different cycle phases that track follicle growth and confirm ovulation. It is painless, requires no special preparation and provides the most accurate data on fertile days.
No — the calendar method as the sole form of contraception has a high failure rate because most women do not have exactly 28-day cycles, and ovulation can occur earlier or later than expected. For reliable protection, we recommend another form of contraception with gynecological consultation.
With an irregular cycle, the calendar method is unreliable. We recommend combining folliculometry, LH test monitoring and basal temperature measurement. A gynaecologist can identify the cause of irregular cycles and suggest appropriate monitoring or treatment.
Sperm can survive in the female reproductive tract for up to 5 days, averaging 3 days. This means intercourse up to 5 days before ovulation can result in conception, widening the fertility window.