From The Sperm And Egg To A Fetus – What Happens In-Between?


Before explaining the process of conception and fertilization, we need to understand what happens during a menstrual cycle and ovulation.

Why Does A Woman Have To Go Through A Menstrual Cycle Every Month?

Our ovaries are quite small, approximately 4 cm in diameter, yet work extremely hard from the time of puberty to menopause. They have to produce mature eggs at each ovulation and also produce the hormones estrogen and progesterone.

A baby girl is born with all her eggs (around 2 million of them) already stored in her ovaries. By the time she reaches puberty, this number has reduced to about 400,000 eggs (oocytes).

Between puberty and menopause only a small percentage of these eggs (±400 to 500) will reach maturity during ovulation and be ready for fertilization.

The reason you have so many eggs is that it is not just one oocyte that develops and matures each menstrual cycle, a number start the process to maturation but only one will dominate and be released at ovulation (ovum is released).

Normally only one ovum is released at ovulation but more than one can occasionally be released such as in non-identical twins.

Ovulation Cycle Chart

The menstrual cycle and ovulation go hand in hand. This ovulation cycle chart shows you the stages of the development of a mature egg (ovum) during your menstrual cycle.

Starting at the top and going clockwise, you can see that inside the ovary the eggs are enclosed in a follicle until the ovum is released at the time of ovulation. The remaining follicle then regresses into a corpus luteum and eventually a corpus albicans (lovely big words!!).

While the immature oocytes are developing, the follicles produce the hormone estrogen.

Once ovulation occurs, the corpus luteum starts producing more progesterone and less estrogen. This is to prepare the lining of the uterus for possible implantation.

If fertilization does not occur, the corpus luteum stops producing these hormones and regresses to a corpus albicans. The thickened lining of your uterus is then shed during menstruation and the cycle starts again and continues until menopause.

Ovulation Cycle

Looking at it from another perspective, this ovulation cycle chart brings together your menstrual cycle (at the top), your hormone levels and what happens inside your uterus during this period, based on a 28 day cycle. You can also see the changes in body temperature during this period.

What Does Ovulation Mean?

Ovulation is when the mature egg or ovum is released from the dominant follicle in the ovary and passes into the fallopian tube. Ovulation is essential for conception to occur.

When Am I Ovulating And What Does Ovulation Feel Like?

Ovulation usually occurs midway through your menstrual cycle, so if you have a regular cycle, it is fairly easy to work out more or less when ovulation occurs. If you have an irregular cycle, you will need to keep accurate records of your menstrual cycle for a few months to establish more or less when ovulation is likely to occur.

A small percentage of women (±20%) develop mild lower abdominal pain or cramps on one side at the time of ovulation. This is called mittelschmerz.

Basal Body Temperature (BBT) measured by a special thermometer first thing in the morning may help you determine when ovulation occurs in your cycle. For this method you will have to be patient and do this for a few months to establish a pattern. What you will find is that your BBT  is lower before ovulation than after ovulation and it does a little dip just before ovulation.

To be more accurate in finding your ovulation date, use an ovulation predictor kit. It is much easier as you just have to pee on a stick and wait to see if you are about to ovulate. It works on the levels of luteinizing hormone (LH) which peaks just before ovulation (see the ovulation cycle chart above).

What Does “Luteal Phase” Mean?

If you look at the top of the ovulation cycle chart, you will see the luteal phase starts after ovulation and continues to menstruation.

Before ovulation, while the follicle is growing, it is called the follicular phase. After ovulation, as the corpus luteum forms, it is called the luteal phase.

During the luteal phase the lining in your uterus normally becomes thicker in preparation for a possible pregnancy.

From Ovulation To Conception

If you are asking the question “When am I most fertile?” the answer is the 12 to 24 hours after ovulation, as this is when the mature egg is ready to be fertilized. After that time the egg will degenerate.

So what happens after ovulation? Either conception occurs and fertilization takes place or the mature egg degenerates and the menstrual cycle continues.

When does conception occur?   Although the mature egg can only survive 12 to 24 hours after ovulation, the male sperm can survive up to 7 days in the fallopian tubes (averaging 3 to 4 days), waiting for a mature egg to fertilize. Conception occurs when a healthy sperm meets a mature egg and form a zygote.

From Conception To Implantation

Once conception has occurred, the process of fertilization continues. Over the next 6 to 7 days the zygote will develop into a morula and then into a blastocyst.

What is a blastocyst? A blastocyst is a sphere with a thin outer layer, a fluid filled cavity and a lining of about 150 cells on the inside.

By this time the blastocyst has reached the uterus and is looking for a suitable place for implantation to occur. Between days 6 to 10 after ovulation, the blastocyst attaches itself to the uterine wall and starts burrowing into it. At the same time the cervix is closed with a mucus plug, creating a safe environment within the uterus.

Conception to Implantation

How long does implantation take? The process of implantation begins after week 1 from conception and continues to the middle of week 2, so it takes a few days.

What Happens After Implantation?

The embryonic stage begins after implantation. This is the process when cells develop into structures such as brain, skin, nervous tissue, organs, muscle and bone, etc. It takes about 8 weeks from conception and development takes place in stages.

For example in the first week after implantation, development of the brain, heart, spinal cord, digestive system and blood vessels begin.

By week 6 the hands and feet are starting to develop.

By week 8, the facial features are becoming distinct and the internal organs are well developed.

After the 8th week from conception, the embryo has all the essential internal and external structures and is now referred to as a fetus until birth.


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