Week 1 of pregnancy – what happens

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This is the first in a series of blogs covering the 40 weeks of pregnancy.

The first week of pregnancy occurs during your last menstrual period. The egg and sperm that will eventually join (or fertilise) to become your baby have not yet fully matured.

Therefore, in week 1 of the pregnancy we explore the maturation of the female egg and male sperm, and then briefly review the future home of the pregnancy, the uterus.

Week 1 of pregnancy – The female egg

Interestingly, a woman’s eggs start their life whilst she is herself an unborn baby, within her own mother’s uterus. As many as 20 million eggs begin to form, but most will degenerate before birth.

As the baby girl is first cuddled by her parents, only 700,000 to 2 million eggs remain within the newborn ovaries.

Yet there is further culling. By the time the young girl reaches the cusp of puberty, and has her first menstrual period, only 400,000 eggs remain in the ovaries.

These are the “potential eggs”, the ones capable of responding to the influx of hormones released from the anterior pituitary in the female brain following puberty, that will enable them to mature and possibly be released from the ovary (ovulation – see week 2 for details). The released egg may then be joined with a sperm (fertilisation – see week 3 for details) to begin the pathway of human development.

Week 1 of pregnancy – Recruiting the egg

So how is one particular egg selected from the awaiting 400,000?

Each month, a hormone in the anterior pituitary of the brain called follicle stimulating hormone (FSH) is released. This hormone acts on the ovary and recruits a small batch of eggs from the waiting thousands, and “activates” them.

Why are some eggs selected each month and not others?

We don’t really know.

There are many theories. One theory is that the activated eggs, by random chance, were more advanced than their peers, and had greater capacity to respond to FSH. Maybe they had a better blood supply, more receptors,or more streamlined internal processes. All we know is that every month, a small group of 5-12 eggs respond to FSH and are recruited for the next menstrual cycle.

Competition continues.

Even amongst the activated 5-12 eggs, only one will eventually ovulate. This selected egg, more advanced by random chance, will become the egg available for fertilisation.

Of course, every now and then a few dead heats emerge in the race and two eggs mature at equal rates and are ovulated, paving the way for non identical twins. Rarer still, three or more eggs may ovulate, leading to the birth of triplets and higher order multiples.

Week 1 of pregnancy -The male sperm

Sperm also take a long time to develop, with the process starting at male puberty.

The male testes begin to secrete the hormone testosterone at puberty. The hormone has many effects, one of which is to promote the growth of the testes and the start of spermatogenesis, or the manufacture of sperm.

A mature sperm consists of a head, a mid piece and a tail. The head contains the nuclei with the genetic material that will become the father’s genetic contribution to the new baby. The mid piece contains the “motor” of the sperm, where energy is generated to enable to sperm to swim through the female reproductive tract and fertilise with the egg. The tail of the sperm contains the propulsion system, that enables the sperm to propel forward on its journey.

Spermatogenesis commences at puberty and continues until a man dies. Sperm are produced in waves, which are synchronised. It takes 64 days to develop a mature sperm.

Week 1 of pregnancy – Recruiting the sperm

After being created in the testes, mature sperm are stored in the epididymis, a coiled duct near the testes. During sex, sperm are ejected into the vas deferens, a communicating channel, supplied with nutritious fluids, and then ejaculated.

During a single ejaculation as many as 200 million sperm may be released. However, only a few hundred sperm survive the journey from the upper vagina, through the cervix and uterine cavity and gain entry into the fallopian tube. The fallopian tube is the ultimate destination of the sperm, as it is the site where fertilisation of the female egg will occur.

Interestingly, the final step to mature sperm doesn’t happen in the male. It happens in the female. As sperm enter the fallopian tube, a chemical reaction between the sperm and female fallopian tube secretions change the sperm and it develops the ability to penetrate an egg and fertilise it. This final step is called “Capacitation”.

Sperm are able to survive and fertilise an egg in the fallopian time for 1-3 days after sex.

Week 1 of pregnancy – Within the uterus

In the first week of pregnancy, the women has her period (or menstrual cycle), shedding the uterine lining (endometrium).

Cramps may accompany the bleeding of the menstrual cycle, as arteries in the uterine lining constrict and deprive the lining tissues of oxygen. This process releases chemicals such as prostaglandins, that trigger painful cramps in some women.This is why medication that blocks the actions of prostaglandins, can help the pain and discomfort of menstrual cramps, which in severe cases is called “primary dysmenorrhoea”.

As the first week ends, the arteries in the uterus begin to open and grow again, sending oxygen, nutrients and hormones to enable a new endometrium to grow over the next month.

Week 1 of pregnancy – Summary

In the first week of pregnancy a woman has her period, or menstrual cycle. The old lining of the uterus is shed, and a new lining begins to grow.

In the mother, a hormone called follicle stimulating hormone is released by the brain, and activates a small group of 5-12 eggs within her ovary.

In the father, sperm were created over the preceding 64 days, and are now stored in the epididymus. Of the 200 million sperm may be released during ejaculation. However, only a few hundred will complete the journey from vagina to the fallopian tube, and undergo capacitation, to enable fertilisation of an egg.

For more reading

Brskov AG. Differentiation of the mammalian embryonic gonad. Physiol Rev 1986, 66: 71.

Clermone Y. Kinetics of spermatogenesis in mammals: seminiferous epithelium cycle and spermatogonial renewal. Physiol Rev 1972; 52:198.

Larsen WJ. Human Embryology 3rd edition. Churchill Livingston, 2002, Pennsylvania.

Wasserman PM. Elements of mammalian fertilisation Vol 1 Basic concepts 1991, CRC Press, Boca Raton, FL.

Larsen WJ. Human Embryology 3rd edition. Churchill Livingston, 2002, Pennsylvania

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2 thoughts on “Week 1 of pregnancy – what happens

  1. I didn’t realise so many eggs were lost before things started. It makes you wonder how the egg is ultimately chosen. Confirms how amazing life really is.

    Liked by 1 person

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