From egg to embryo

Having a baby is a such a natural part of life that we tend to take it for granted. But the act of getting pregnant—creating a life inside of you—is nothing short of a miracle. The number of intricate and complex steps that needs to flow in seamless harmony is truly fascinating. However, it is not unusual for some of these steps to be out of sync. This is when getting pregnant becomes more complicated and difficult for some women.

Let’s look at this amazing journey from egg to embryo.

It all begins with the egg 1

Did you know that usually, a woman is born with around one to two million eggs in her ovaries? However, around 11,000 of these eggs die before puberty, and after puberty, a woman would generally lose around 1,000 eggs each month. Which means in her usual lifespan, a woman has around 300,000 to 400,000 eggs which she releases during ovulation. And from these, only one mature egg is destined to ovulate each month.

The power of ovulation 2

What is ovulation?
Remember those 1000 eggs that you generally lose every month? Well, using follicle-stimulating hormones, all of these eggs begin to develop and mature for around 14 days. Until one dominant egg becomes fully mature, and the follicle surrounding the egg bursts, releasing this mature egg from one of your ovaries. This process is called ovulation, and is an integral part to becoming pregnant.

When does ovulation happen?3
Ovulation usually happens around day 14 of a 28-day menstrual cycle. This happens when a sudden surge in hormones causes the ovary to release its egg.

Facts about ovulation4-6

  • Ovulation usually occurs about two weeks after the start of your last period.
  • Ovulation, or the release of an egg, can occur any time between the 11 th and 21 st day of a regular menstrual cycle.
  • An average menstrual cycle is anywhere between 28 to 32 days.
  • Because every period is not text book, you can try calculating your ovulation date by monitoring your period for a few months, and then pinpointing it down to the middle of your cycle.
  • If not, you can get an ovulation prediction kit over-the-counter at a pharmacy. OPKs help to predict the presence of LH (luteinizing hormone) in your urine.

Once released from your ovary, the mature egg needs to make an approximate 30-hour descent through the fallopian tubes down into the uterus. During this descent, your body will produce more progesterone, a female hormone, in order to help prepare the uterine lining for a possible pregnancy.

Fertilisation can happen when fertile sperm swimming up through the reproductive tract meet the mature egg travelling down towards the uterus. This usually takes place in whichever fallopian tube the egg is descending through.

Generally, a mature egg can only be fertilised in the 12-24 hours after its been released from your ovary. However, sperm can live in the reproductive tract under ideal conditions for up to five days. If you are trying to get pregnant, having sex during the six days leading up to and including ovulation is ideal. This is called the ‘fertile window’.

How does fertilisation happen?8

Fertilisation generally happens while the egg is slowly descending through the fallopian tubes and sperm are energetically swimming up through the reproductive tract towards the uterus. Whilst hundreds of sperm will try to penetrate the egg’s hard outer layer, only one single sperm will accomplish this task. Immediately after, the egg undergoes a chemical reaction to prevent other sperm from penetrating it.


As the fertilised egg continues its descent, it begins to divide into cells. Two, then four, and on and on, until around a week later, the egg reaches the uterus as a growing cluster of around 100 cells called a blastocyst. Implantation is when this blastocyst, which started out as the fertilised egg, attaches itself to the lining of the uterus.

Extra boosts of oestrogen and progesterone help the lining to thicken, which provides the nutrients needed for the blastocyst to grow and become a baby.

But it doesn’t end there. The cells continue to divide; some go on to develop into your baby, while others form the nourishment and oxygen supply structure called the placenta. While all this is happening, hormones are released to signal your body that a baby is growing inside the uterus. These hormones are also responsible in alerting the uterus to maintain its lining rather than shedding it (during a menstrual cycle). This means you will not have your period that month, and is usually one of the first—and most common ways—of realising you are pregnant.


  • Being overweight or severely underweight may reduce your chances of ovulating and getting pregnant. Obesity can also cause infertility and low testosterone in men.
  • Age is another factor that can affect your chances of getting pregnant. Consult your doctor if you are under 35 and have been trying to conceive for more than 12 months, or over 35 years and have been trying to get pregnant for over six months.
  1. Sherman J. Silber, M.D. Beating your biological clock – How it works. Available at: Accessed: June 2020.
  2. Rogel Cancer Center. Normal ovarian function. Available at: patients/normal-ovarian-function. Accessed: June 2020.
  3. Cleveland Clinic. Pregnancy: Ovulation, Conception and Getting Pregnant. Available at: Accessed: June 2020.
  4. OnHealth. The Journey from egg to embryo. Available at:
  5. OnHealth. Facts to help you get pregnant. Available at:
  6. HealthLine. What is Ovulation? 16 things to know about your menstrual cycle. Available at: ovulation#:~:text=Ovulation%20is%20a%20part%20of,to%20develop%20into%20a%20pregnancy.
  7. University of California San Francisco Health. Conception: How it works. Available at: works#:~:text=Adhesive%20sites%20on%20the%20cilia,tube%20takes%20about%2030%20hours.
  8. What to expect. How fertilization happens. Available at: happens.aspx