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Conception and testing for pregnancy

Conception and testing for pregnancy

Conception is quite the miraculous process.  Here we explain how conception occurs, when each step happens and how you can test for pregnancy.

A new baby is conceived when a woman releases a mature egg at 'ovulation' (about 2 weeks before her next period is due). Her egg is fertilised by one of the many million sperm produced by the man, ejaculated during sexual intercourse. About 3 to 5 days before ovulation a woman starts to produce special, thin, slippery fertile mucous (resembling egg white). This fertile mucous sits in her cervix and lines her uterus and fallopian tubes. If she has sex while this mucous is present, the man's sperm can swim through the mucous, drawing the sperm up into the woman's fallopian tubes, to 'lie in waiting' for her to release an egg. This fascinating process, including the male and female reproductive system, are explained in depth here

Conception usually takes place in one of the woman's fallopian tubes (depending on which ovary releases a mature egg). When the egg is released into the adjacent fallopian tube, thousands of waiting sperm (that have survived the journey) surround it. A group of the stronger sperm start to burrow with their heads into the egg's thick outer covering (called the 'zona pellucida'). The head of each sperm release a special enzyme (called 'acrosin'), which allows their head to create an opening in the egg's zona pellucida. When the most determined sperm reaches the inner part of the egg, it breaks through into the egg's centre. As soon as this happens a chemical reaction takes place, automatically changing the egg's covering to become impenetrable to any other sperm. At the same time the thrashing tail of the successful sperm detaches, leaving only its head within the egg.

About half of a man's sperm carry the genes to form a girl baby (known as the X chromosome) and the other half of his sperm carry the genes to form a boy baby (known as the Y chromosome). Therefore, it is the man who determines the sex of each child. The man's 23 chromosomes in his single sperm (all his genetic coding) and the woman's 23 chromosomes in her egg (all her genetic coding) combine and shuffle both parents' genes to create a unique person with a varied genetic make-up of 46 chromosomes. Nature encourages this 'mixing of different genes' to help the next generation adapt to our changing environment, as the human race evolves.

NOTE: When conceiving twins (triplets or more) the woman usually releases 2 (or 3 or more) eggs at once. These eggs are then fertilised to create non-identical twins (or triplets etc.) The woman's ability to produce more than one mature egg at a time is something that is inherited (and why non-identical twins are more common, because they 'run in the family'). Identical twins only occur after fertilisation takes place. The fertilised egg (with all the combined genes from both parents) suddenly splits into two identical developing babies. This is a twist of fate that happens less frequently, but is possible for any couple, because it is not something that is inherited. You can read more in twins, triplets or more - the pregnancy.

When the sperm enters the egg they fuse and combine their genes to make one cell. The fertilised egg is then known as a 'conceptus'. Within hours of the conceptus forming it splits into 2 identical cells.

Hours after the conceptus splits into two cells, they split again to create 4 cells.

This process of creating new identical cells continues, so that by the 4th day after fertilisation the conceptus becomes a group of about 16 to 20 cells, called a 'morula' (or a 'ball of cells').

On the 5th day after fertilisation, most of cells in the morula start to move to one side, creating a bunch of cells inside a sac of fluid (that the baby will float in). The individual cells (which all previously looked the same) now start to change (or 'differentiate') as they begin preparing to perform their individual functions, now destined to become the different parts of the baby's body (and the placenta). As cell differentiation continues, the 'morula' becomes what is known as the 'blastocyst' by day 6 after fertilisation.

While the conceptus is growing into a blastocyst it is also gently being moved down the woman's fallopian tube towards her uterus. This is achieved by hundreds of small, hair-like structures called 'cilia' (that line the tube), in combination with muscle fibres in the walls of the tube, producing wave-like contractions. By the time the conceptus becomes a blastocyst (taking about 6 days) it enters the woman's uterus to implant in its thick lining (about a week before the woman's next period would have been due).

The blastocyst takes another 4 to 6 days to fully embed or 'implant' in the woman's endometrium. The implantation process takes about 6 days to complete (finishing about 12 days after fertilisation, or about 2 days before the woman's next period would have been due). Once the blastocyst is fully implanted, the mother's body starts to provide the developing baby with nourishment to continue to grow. Up until this time the baby has been nourished by glucose secreted from the lining of the fallopian tubes and the uterus.

NOTE: While implantation cannot usually be felt, some women will experience a small amount of bleeding as their baby completes the 'implantation' process into the thick uterine lining. Because an implantation bleed can happen around 8 to 12 days after ovulation (or about 2 to 6 days before the woman's next period would have been due), it can often be confused with having a 'period' (although an implantation bleed is usually lighter and does not last as long as a normal period). This can make the woman believe she is not pregnant (for a while anyway) but it can also contribute to miscalculating the baby's due date. This is because the 'due date' is estimated using the starting date of the last normal menstrual period. Caregivers will try and confirm that the woman's last 'period' WAS definitely normal, so that the due date is calculated from the last period, not the implantation bleed. (Otherwise the baby's due date will be estimated as being about 3 to 4 weeks later than it should be).

During the 6 days it takes the blastocyst to implant into the lining of the woman's uterus, the cells continue to multiply and differentiate, lining up to form a flat disc of cells with 3 distinct layers. These layers are called the:

  • Endoderm, which will become the baby's internal organs, including their lungs, liver, bowel and bladder.
  • Mesoderm, which will become the baby's skull and bones, sex organs, muscles and heart.
  • Ectoderm, which will become the baby's skin, hair, eyes, ears, brain and spinal cord.

By 14 days after fertilisation (or when the women's period would have been due) the blastocyst becomes an 'embryo', implanted firmly into their mother's uterine lining. The embryo has all the vital building blocks in place to create an individual human being, and you don't even know you are pregnant yet! 

Pregnancy tests

After conception takes place, your developing baby will take about 6 days to move down your fallopian tube and into your uterus. It then takes at least a further 6 days (or more) for your baby to fully implant into the thick lining of the uterus. Therefore, it is not until at least 8 to 12 days after ovulation (or 2 to 6 days before your next period would have been due) that your baby will start to physically interact with your body. This 'interaction' involves your body providing your baby with nourishment through your blood stream as well as your baby producing hormones that move into your body, to support the pregnancy.

Once your baby fully implants, they start to release a hormone called 'human chorionic gonadotrophin hormone' (or 'HCG') into your blood stream. The HCG level initially starts off very low, but rapidly increases (doubling every second day), so that within a few days to a week or so, the HCG level becomes high enough to be detected in your blood stream and urine. It is this increasing level of HCG that forms the basis of blood and urine pregnancy tests. This is also why pregnancy tests do not show up as being 'positive' until the woman's period would have been due (and why women do not start to feel the physical signs of pregnancy until after this time).

NOTE: Urine pregnancy tests only test for the one hormone, 'HCG'. Other hormones increase during pregnancy as well, mainly progesterone and some oestrogen, which contribute to many of the early physical signs of pregnancy. These hormones are not tested for with pregnancy tests. Progesterone rises pre-menstrually, but continues to rise further after the period is missed. This is why pre-menstrual feelings can be confused with early pregnancy signs. Most women don't get pregnancy signs until after their period is missed and the hormones increase to levels that their body is not used to. However, a few women do get pregnancy signs earlier, especially if they have had a baby before, because their body is more sensitive to the hormones and they know they 'feel different' than just pre-menstrual.

After implantation, HCG hormone rapidly increases in a pregnant woman's blood stream, peaking to its highest levels between 8 and 11 weeks of the pregnancy. It then slowly decreases until about 17 weeks of the pregnancy, remaining low until several weeks after the birth of the baby. Once the HCG levels start to lower, the maturing 'placenta' takes over the role of producing hormones to support the pregnancy (at around 12 weeks).

Pregnancy tests to detect HCG can be performed using 2 methods:

Urine pregnancy test

Modern urine pregnancy tests rely on an 'immunoassay' testing technique to detect the presence of 'human chorionic gonadotrophin hormone' (or 'HCG') in a pregnant woman's urine. Urine pregnancy tests can usually be purchased in supermarkets and at pharmacies to use at home. Similar tests may also be used by caregivers (such as your local doctor or woman's health nurse) during a medical check to detect a pregnancy, or as a secondary confirmation of your home pregnancy test.

In recent years, urine pregnancy tests have become more refined and much more accurate. However, they need to be used exactly as directed by the manufacturers, and even then their reliability can depend on other factors, being:

The timing of the test

The level of detectable HCG hormone in a pregnant woman's urine will depend on how far pregnant she is. In most cases, it is possible for a urine pregnancy test to show 'positive' from around the time that the next period would have been due (but not earlier than this). However, a common problem with urine tests is that in individual women, the HCG levels may not be high enough until a few days (or up to a week or so) after the period was due. This means that you may perform an early urine pregnancy test and get a 'negative' result, even though you ARE pregnant. If the test is repeated a few days later (when the HCG levels are higher), the test should show 'positive', presuming that the pregnancy is continuing.

The concentration of the urine

The level of detectable HCG hormone in a pregnant woman's urine will also depend on how concentrated her urine is. If the pregnancy is well advanced (more than 5 to 6 weeks, or 1 to 2 weeks after the missed period) then the sample of urine used can be taken at any time of day and will still show as being 'positive'. However, if the pregnancy test is performed early (around the time the period is due) or even in the few days following, the test may only show 'positive' if a concentrated urine specimen is used. Urine passed first thing in the morning (after waking) is usually more concentrated than urine passed during the rest of the day. By using the first urine sample of the morning, the pregnancy test will tend to be more reliable.

If a pregnancy test is used as directed and done at an appropriate time (when the period is overdue) with a concentrated urine sample, they can be up to 97% accurate. However, if the test is done incorrectly, too early and/or with a weak (or dilute) urine sample, they are thought to be about 75% accurate.

Early miscarriage

Sometimes a woman will perform an early pregnancy test, which shows a 'positive' result. Then in the days following a period arrives. This is really an early miscarriage. In many cases, women are not aware of early miscarriage, because they do not perform a pregnancy test and regard the miscarriage as a late period.

Baby implants later

It can take longer than 12 days for some babies to fully implant in the uterus. Studies have shown that up to 10% of women will not have a 'positive' urine pregnancy test until several days after their period would have been due.

Doing the test

There are many different types of urine pregnancy tests available on the market. However, they all tend to work on the same principles. Some will come as a single test, while others will provide two tests. The second test may come in handy if your first test result is 'negative', and you need to repeat the test in a few days time.

When you purchase your urine pregnancy test you will be given instructions on how the put your sample of urine onto the test. Make sure you follow the manufacturer's instructions. As mentioned before, the first urine passed in the morning will usually be more reliable (unless the pregnancy is quite advanced). Once the urine soaks into the blotting paper (or similar) on the test, a result will be visible within minutes.

Most tests will have 2 coloured markings to look for (usually as a dot or line in any colour, usually pink, red or blue). If only one marking becomes visible the test is regarded as NEGATIVE and you are not pregnant OR you may be doing the test too early in the pregnancy and/or your urine may not be concentrated enough. Therefore if your period still does not arrive within the next few days, repeat the test. 

If 2 markings become visible (even if the second marking is very faint) then your pregnancy test is POSITIVE and you are pregnant - Congratulations!

NOTE: It is very rare for a pregnancy test to show 'positive' if you are not pregnant.

Blood pregnancy test

The blood test to detect a pregnancy is called a 'beta HCG'. It is possible for a blood test to show a 'positive' pregnancy result as early as 8 to 12 days after fertilisation (or 2 to 6 days before the next period would have been due). However, blood tests tend to be more accurate around the time the next period would have been due (similar to urine tests). The beta HCG needs to be at least 5 IU/L to be regarded as a 'positive' result. However, this may only indicate that a pregnancy is possible, or has recently existed (particularly if the woman is bleeding and a miscarriage is being questioned). If the test is repeated in 2 to 3 days and the beta HCG has at least doubled, then this will confirm that the pregnancy is continuing. A beta HCG of 25 IU/L (or more) is definitely regarded as a 'positive' pregnancy test. 

Blood tests for pregnancy tend to only be used by caregivers if it is important to detect a pregnancy very early, particularly if the woman has a health condition in which medical treatments need to be adjusted (such as diabetes or epilepsy) or it is possible the pregnancy may be ectopic. They are also used to help diagnose a miscarriage. If a woman suspects she may be pregnant a blood test may be performed to help make decisions about prescribing medications, performing an X ray or proceeding with an elective operation (and whether there are risks involved in exposing the developing baby to these). You can read more about HCG blood tests in tests and treatments for miscarriage.



Read more about early pregnancy

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