Category Archives: Reproductive physiology

Since a woman can never know when she has ovulated, working out the exact point of fertilisation is also impossible. What we do know is that implantation occurs 6-12 days after fertilisation.¹ Once implantation is complete, pregnancy is established.²

Pregnancy begins when a fertilised egg has been implanted in the wall of a woman’s uterus. This definition is critical to distinguishing between a contraceptive that prevents pregnancy and an abortifacient that terminates it. ^2,3

When women have unprotected intercourse, they are not immediately pregnant. Pregnancy can only occur a minimum of 6 days after intercourse (when a fertilised egg implants in the uterus).

Many women don’t understand when pregnancy begins. They believe it starts the moment they had UPSI. Within 5 days of UPSI a woman cannot be pregnant from that UPSI, because implantation cannot yet have occurred. But this mistaken understanding means that they can feel guilty about using EC because they wrongly believe it to be a form of abortion.⁴

After UPSI sperm can survive for approximately 5 days within the female reproductive tract.¹ This means that during the average woman’s menstrual cycle there are six days when intercourse can result in pregnancy; this ‘fertile window’ is the five days before ovulation plus the day of ovulation.²

So when is the fertile window? Current evidence challenges the simplified ‘text book’ understanding of the menstrual cycle.² We now know that only about 12% of ovulations happen on day 14.³

The variability of ovulation is large – it can happen from day 11 to day 21.² Because sperm stay viable for up to 5 days¹, the period over which conception is likely to occur runs from day 6 to day 21 for regularly cycling women.² If the cycle is not regular, there is a risk of ovulation happening even later in the cycle.² The conception risk period does not end before day 28 of their cycle.² This shows that there is no such thing as a risk free period.²

Ovulation also varies from cycle to cycle.²

Although the risk of pregnancy exists most of the time,² women may underestimate the risk of pregnancy.⁴ This lack of awareness of pregnancy risk may be the most important barrier to EC use⁵

The highest risk of pregnancy is when ovulation happens shortly after UPSI⁶

Sperm viability declines over time. This means that the risk of conception is highest during the first three days following unprotected sex or contraceptive failure.⁶

Therefore, to avoid unwanted pregnancy, it is critical to avoid ovulation (happening shortly after UPSI while the sperm is still viable) by using EC as soon as possible.

The follicular phase (stages 1-4 on the diagram below)

The follicular phase starts on the first day of menstruation and ends with ovulation.¹ Prompted by the hypothalamus, the pituitary gland releases follicle stimulating hormone (FSH).² This hormone stimulates the ovary to produce several follicles (tiny nodules or cysts), on the surface.¹ Each follicle houses an immature egg.¹ Usually, only one follicle will deliver an egg, while the others die.¹ The growth of the follicles stimulates the endometrium to thicken in preparation for possible pregnancy.¹

The ovulatory phase (stage 5 on the diagram below)

Ovulation is the release of a mature egg from the ovary’s surface in response to rising levels of luteinising hormone (LH) and FSH.¹ When the LH reaches a peak it triggers the rupture of the developing follicle to release the mature egg: ovulation; with no LH surge, ovulation does not occur.¹ The released egg is funnelled into the fallopian tube and towards the uterus by waves of small, hair-like projections. The life span of the typical egg is only around 24 hours.³ Unless it meets a sperm during this time, it will die.

The luteal phase (stages 6-8 on the diagram below)

Upon the release of the ovum, the ruptured follicle stays on the surface of the ovary. The follicle transforms into a structure known as the corpus luteum, which releases progesterone and small amounts of oestrogen.^1,2 The thickened lining of the uterus is maintained and waits for a fertilised ovum to implant.¹ If this happens the implanted ovum will start to produce human chorionic gonadotropin, detectable in a urine test for pregnancy.⁴ If pregnancy doesn’t happen, the corpus luteum regresses, usually around day 22 in a 28-day cycle.¹ The drop in progesterone levels causes the endometrium to break down and menstruation begins again.¹

The highest risk of pregnancy is when ovulation happens shortly after UPSI6

Therefore, to avoid unwanted pregnancy, it is critical to avoid ovulation (happening shortly after UPSI while the sperm is still viable) by using EC as soon as possible.

The highest risk of pregnancy is when ovulation happens shortly after UPSI⁶