Understanding Fertility

Fertility is described as an individual’s natural ability to successfully conceive a child. There are a number of influencing factors that may affect the chance of achieving a successful pregnancy. To give yourself the best chance of conceiving, it can be useful to understand the fundamentals of reproduction.
Here you will find information about the female reproductive system, the menstrual cycle and how to track it, as well as the role hormones play in fertility.

Reproductive system

Physiology and function of the female reproductive system

The female reproductive system is in charge of storing gametes (i.e., eggs), producing sex hormones, and maintaining optimal conditions for an embryo to develop.¹ It consists of: Ovaries, Uterus, Fallopian Tubes, Cervix and Vagina.

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Adapted from: Elad et al., Physiology 2020.⁴

The fertility cycle

The fertility, or menstrual, cycle is made up of two phases that typically take 28 days to complete, although this can vary from person to person. The length of a cycle can be calculated from the first day of a period to the day before the next period starts. The cycle is separated into two distinct phases, the follicur phase and luteal phase, with ovulation occurring midway through the cycle.¹

The follicular phase

The follicular phase begins with menstrual bleeding and ends right before a surge in luteinising hormone. It starts on the first day of menstruation, where bleeding lasts 4-6 days. Other common symptoms include cramps, bloating, tender breasts, and headaches. Menstrual bleeding is caused by a change in hormones due to an egg released during the previous cycle not being fertilised. The thickened lining of the uterus begins to shed, leading to the release of blood, mucus, and tissue – also known as the monthly period. The pituitary gland, located in the brain, releases a hormone called follicle-stimulating hormone (FSH). FSH stimulates the ovaries to produce 5-20 fluid filled sacs called follicles, each containing an immature egg; these follicles are found in the outside layer of the ovaries. Typically, only one egg matures and is released from the ovaries per cycle. Meanwhile, the level of oestrogen released by the ovaries will rise, causing the uterus lining to thicken in preparation for a possible pregnancy.

Ovulation

The luteinizing hormone (LH) surge takes place midway through the cycle (day 15), it marks the end of the
follicular phase and the beginning of the luteal phase. 24-48 hours later, ovulation occurs on average at day 17, although this will vary depending on the length of the cycle.⁶ Rising oestrogen levels during the follicular phase trigger the brain to release the hormone LH. This causes the mature egg to be released from the ovary and enter the fallopian tube, to be fertilised by sperm.⁵

The luteal phase

This is the second stage of the menstrual cycle. It normally lasts around 14 days and may be accompanied by premenstrual syndrome (PMS) symptoms, such as headaches, mood changes, and food cravings.⁵
Once the follicle has released the matured egg to be fertilised, it develops into a new structure called the corpus luteum (yellow body). After ovulation, the corpus luteum continues to develop in the ovary at the site of the ruptured follicle that released the mature egg. The corpus luteum is a temporary endocrine structure involved in the production of relatively high levels of progesterone which helps to thicken the lining of the uterus and prepare the woman’s body for the potential implantation of a fertilised egg.⁵
If fertilisation occurs and the embryo implants into the uterus, the placenta will start producing human chorionic gonadotropin (hCG) – the hormone detected by pregnancy tests to indicate a positive result.⁷ HCG also signals the corpus luteum to continue progesterone secretion, thereby maintaining a thickened uterus lining, providing a favourable environment for the fertilised egg to develop into an embryo.⁵
From this point on, the corpus luteum is called the corpus luteum of pregnancy, which continues to function until around week 12 of pregnancy. If the egg is not fertilised, the corpus luteum stops secreting progesterone and decays after approximately 10 days. The progesterone drops 2-3 days before the end of the luteal phase and the beginning of menstruation. Oestrogen and progesterone levels decline dramatically, which triggers the beginning of the menstrual phase (i.e., a period).⁵

Getting pregnant

Signs that you are ovulating

When trying for a baby, ovulation is the most important event to consider. Ovulation is the moment the follicle bursts and releases the egg cell into the fallopian tube, where it waits to be fertilised by sperm. It is during the first 24 hours of the luteal phase that a woman is most fertile, and pregnancy is most likely to occur.⁴

Therefore, becoming familiar with ovulation and when it is likely to occur can help improve the chances of getting pregnant.⁶ Ovulation test strips are urine-based tests that can be used at home to detect upcoming ovulation. They work by detecting the hormone LH.⁸Sometimes they are called OPKs, ovulation predictor kits, or simply ovulation tests.

Some Common signs of ovulation are:

Adapted from: Gurevich R, 2020.⁸

Fertility process

After the egg has been fertilised by a sperm cell, it travels along the fallopian tube to the uterus and continues to divide. The embryo reaches the uterus six days after fertilisation and implants into the thickened endometrium lining where it will continue to grow.⁹

The outer cells of the embryo will form the placenta⁹ and produce human chorionic gonadotropin (hCG).⁷ This is when pregnancy begins and can be detected with a pregnancy test.

Adapted from: Dey SK, J Clin Invest 2010.³

How long does it take to get pregnant?

Whilst some couples will conceive within a year of regular sexual intercourse, for other couples it can take longer. It’s not uncommon for it to take a year or more to become pregnant through natural conception; in fact, 1 in 6 couples have difficulty conceiving within this timeframe.¹⁰

Fertility cycle tracking

When trying for a baby, fertility cycle tracking can be a useful way to predict the ovulation period – also known as the fertile window – and therefore when a woman has the highest chance of conceiving. There are a number of period tracker apps available that allow the user to log details of their menstrual cycle every month, such as the timing of menstruation, body temperature, and changes in cervical fluid, which can all be important indicators of ovulation.⁶Alternatively, ovulation can be monitored using an ovulation predictor kit. This is an accurate method for determining when ovulation may occur through the measurement of LH in urine, which can indicate when the woman is at their most fertile.⁶

Fertility hormones

Hormones and the fertility cycle

Hormones play a significant role in the regulation of the reproductive system, from helping the egg to mature, travel to the uterus, and safely implant as an embryo in the uterus lining. If hormone levels are too high or too low, it may have an impact on an individual’s fertility. Read on to learn about the important female fertility hormones and the changes to expect during pregnancy.

Female fertility hormones

Hormones released by the ovaries:

Oestrogen

Oestrogen is the primary female sex hormone. There are three types found in women: oestradiol, oestriol and oestrone, oestradiol is the predominant form found in the body. Oestradiol levels are highest during ovulation. It plays an important role in the maturation and release of an egg from the ovaries, as well as the thickening of the uterus lining for the implantation of an embryo.^2,11-13

Progesterone

Progesterone is commonly known as the ‘pregnancy hormone’. During the menstrual cycle, progesterone rises during the luteal phase after being released from the corpus luteum. Here, progesterone prepares the uterus for pregnancy by triggering the uterus lining to thicken. If the released egg is not fertilised, progesterone will fall, and a new menstrual cycle begins.^11,14

Hormones released by the brain:

Follicle-stimulating hormone (FSH)

FSH plays a crucial role in ovarian function. During the follicular phase of your menstrual cycle, FSH causes your follicles to mature, leading to the development of a dominant follicle. Levels of FSH peak prior to ovulation (at the same time as the LH surge) and will remain low during the luteal phase.^11,15

Luteinising hormone (LH)

LH is one of the main hormones involved in the menstrual cycle. Rising oestradiol levels trigger the release of LH from the brain, causing the release of the mature egg from the follicle and therefore instigating ovulation. LH also stimulates the corpus luteum to release progesterone, a hormone required to support the early stages of pregnancy.^11,16

What happens to your hormones during pregnancy?

During pregnancy, the female body will experience various changes – many of them caused by fluctuations in hormone levels:

Changes to human chorionic gonadotropin (hCG):

During pregnancy, a hormone called hCG is produced in abundance by the placenta.⁷ This is the hormone that pregnancy tests detect in the urine to indicate a positive test.¹⁷ It plays an important role in the production of oestrogen²³ and progesterone,⁷ both vital in supporting the growing baby.

Changes to oestrogen

Oestrogen (in the form of oestriol) produced by the placenta steadily increases throughout pregnancy,¹⁷ until it peaks in the third trimester. Oestriol stimulates growth of the uterus and helps to prepare it for birth.¹⁸

Changes to progesterone

Progesterone has a vital role to play during pregnancy, acting as a muscle relaxant to prevent the uterus from contracting until labour^19,20 and helping to maintain a supportive environment for the growing foetus.²¹ It is initially produced by the corpus luteum in the early stages of pregnancy before the placenta eventually takes over as the main source at around 12 weeks of pregnacy.²²

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