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Pregnancy Day by Day : Welcome to your First Trimester (part 7)

- 9 Bad Habits That Can Cause Miscarriage
- How to have natural miscarriage
- Foods That Cause Miscarriage
- Signs Proving You Have Boy Pregnancy

Your 3rd Week

If you ovulated and the egg met a sperm, amazing things will happen fast. It takes just three days from fertilization for a single egg to divide into a ball of 58 cells. By the end of the week, this ball, called the blastocyst, will have reached the uterus, where it will start to implant in the lining. It will be a couple of weeks before you know whether you’ve conceived, but special hormones kick in now to help maintain the pregnancy.

NOTE

This is the week a miracle takes place—Your baby is conceived

Fertilization

You are: 2 Weeks and 1 Day 265 days to go…

Your newly released egg will only survive 24 hours, but hopefully in that time it will meet sperm and be fertilized.

What’s happening inside

Here the egg is shown surrounded by sperm. Although only one sperm will fertilize the egg, several hundred are thought to be necessary to break down its defensive layers and enable fertilization to take place.

You are likely to have ovulated and your unfertilized egg now begins its journey. Once it has been released by the ovary, the egg is swept up by one of your fallopian tubes and, moving in the direction of the uterus, comes to rest in the widest portion of the tube, awaiting fertilization.

It is no exaggeration to say that for each sperm released the chance of even reaching the site of fertilization is in the order of one in a million. Around 300 sperm reach the tube but only one will fertilize the egg. Once the sperm has penetrated, it triggers a reaction that makes the surface impenetrable. Each sperm and egg contain 23 chromosomes, half of the total genetic material required. The egg will always contain an X chromosome but the sperm will carry either an X or Y chromosome and therefore determines the sex of the embryo. The sperm and egg chromosomes combine forming the “zygote” and fertilization is complete.

A few hundred sperm survive the journey and encounter the egg in the fallopian tube, but it is just one sperm that actually fertilizes the egg.

… Dads
Fit but not fertile?

If you want to become a dad, there are many reasons why you should ensure you’re in good shape, not least to support your partner as she prepares for pregnancy. However, while a couch potato lifestyle isn’t desirable for men who want to conceive, it seems that pulling out all the stops at the gym might not be the best course of action either.

Researchers asked a group of fit young men to exercise intensively four times a week for two weeks. Afterward, their semen was tested and found to contain fewer sperm and lower levels of the hormones essential for conception. These hormonal changes were temporary and returned to near normal within a few days of the men resuming previous levels of activity.

The concern is that recovery might not be so fast among older men, or in those who have poor sperm counts and/or low hormone levels. So stay in shape but don’t overdo it.

The hormones responsible for the production of sperm are released every 60 to 90 minutes. So a man is constantly producing sperm cells.

In theory, this means that a male is always fertile, but it takes sperm a 72-day period to fully develop. So leading an unhealthy lifestyle during that time will impinge on the quality. For this reason, if you’re trying to conceive, your partner should embark on a healthy lifestyle for three months to produce good sperm.

Genes and Inheritance

The genes that parents pass on to their children at least partly determine their children’s physical and mental characteristics. In some cases, an abnormal gene may be passed on, resulting in an inherited genetic disorder.

What are genes?

Genes are located on rodlike structures called chromosomes that are found in the nucleus of every cell in the body. Each gene occupies a specific position on a chromosome. Because genes provide instructions for making proteins, and proteins determine the structure and function of each cell in the body, it follows that genes are responsible for all the characteristics you inherit.

The full genetic instructions for each person, known as the human genome, is carried by 23 pairs of chromosomes, and consists of around 20,000–25,000 genes.

How inheritance works

At conception, the embryo receives 23 chromosomes from the mother’s egg and 23 chromosomes from the father’s sperm. These pair up to make a total of 46 chromosomes. Pairs 1 to 22 are identical or nearly identical; the 23rd pair consist of the sex chromosomes, which are either X or Y. Each egg and sperm contains a different combination of genes. This is because when egg and sperm cells form, chromosomes join together and randomly exchange genes between each other before the cell divides. This means that, with the exception of identical twins (see How twins are conceived), each person has unique characteristics.

How gender is determined

Of the 23 pairs of chromosomes that are inherited, one pair determines gender. This pair is composed either of two X (female) chromosomes, in which case the baby will be a girl, or of one X and one Y (male) chromosome, in which case the baby will be a boy.

An egg always contains one X chromosome, while a sperm can carry an X or a Y chromosome. Whether your baby is a boy or a girl will therefore always be determined by the father. If a sperm carrying an X chromosome fertilizes the egg, the resulting embryo will be a girl. If a sperm with a Y chromosome fertilizes the egg, the resulting embryo will be a boy. In the male, both the X and Y chromosomes are active. In females, however one of the two X chromosomes is deactivated early in development of the embryo in order to prevent duplicate instructions. This could be the X chromosome from either the mother or the father.

Gene variations

Each gene within a cell exists in two versions, one inherited from each parent. Often these genes are identical. However, some paired genes occur in slightly different versions, called alleles. There may be two to several hundred alleles of a gene, although each person can only have two. This variation in alleles accounts for the differences between individuals, such as color of eyes or shape of ears. One allele may be dominant and “overpower” the other recessive one (see Dominant and recessive genes).

Why genetic disorders occur

Genes usually exist in a healthy form, but sometimes a gene is faulty. Genetic disorders arise either when an abnormal gene is inherited or when a gene changes, or mutates. Genetic disorders may follow a dominant or recessive pattern of inheritance (see Dominant and recessive genes). They can also be passed on via the X chromosome. Such sex-linked disorders are usually recessive, which means that a woman can carry the faulty gene without being affected, because she has another healthy X chromosome to compensate. If a boy receives an affected X chromosome, he will be affected; a girl will be a healthy carrier like her mother. An affected male could pass on the affected gene only to his daughters.

How genes are passed on
Through the generations

Through each generation, genes are shuffled and re-shuffled. Half of a baby’s genes come from its father and half from its mother. The baby’s parents in turn inherited half each of their genes from each of their own parents. One quarter of each person’s genes therefore come from the grandparents. So how does this happen? Instead of containing the full complement of 46 chromosomes, each egg and each sperm has just half, or 23, chromosomes each. When they meet at conception the chromosomes pair up to again make up the full complement that now forms the genetic blueprint for the new individual. One of the 23 pairs of chromosomes are sex chromosomes, so gender is also determined at conception. Each egg carries an X chromosome and each sperm either an X or Y. If two X chromosomes combine, the baby will be a girl; if an X and Y chromosome combine, the baby will be a boy (see How gender is determined).

Genetic inheritance means that successive generations can share certain characteristics.

Patterns of inheritance
Dominant and recessive genes

Genes come in pairs, and each gene in a pair may differ slightly. One gene may be dominant, and override the other gene, which is recessive. A recessive gene only has an effect if both genes in a pair are recessive. An example is eye color, although this is not as simple as depicted here.

You are 2 Weeks and 2 Days 264 days to go…

When your egg has been fertilized, hormonal changes naturally occur to stop your normal menstrual cycle.

What’s happening inside

Here a human egg cell 24 hours after fertilization is artificially colored purple. Around the egg is a thick layer (yellow) that has now become impenetrable. The two red areas, or pro-nuclei, contain genetic material from the mother and father before it has fused.

At this early stage following fertilization of your egg, the developing embryo will signal its existence to the pituitary gland in your brain and switch off your menstrual cycle. It does this by producing a new hormone, called human chorionic gonadotrophin (hCG). This hormone overrides your usual monthly cycle and maintains the high progesterone levels that are essential for your pregnancy. The hormone progesterone (see This is Day 4 of your Menstrual Cycle) is essential to an embryo’s survival in the uterus, and therefore to your baby’s well-being and development before birth.

Later, starting around weeks four to five, your embryo will make all the hormones needed to maintain its own life. Of course its nourishment and shelter come from you, but even in the very early weeks of pregnancy the embryo behaves like an independent human, at least as far as its hormones and genes are concerned.

… Doctor
Q: I’ve been doing ovulation tests, but they’ve all been negative. Does this mean I haven’t ovulated this month?
A: Even if you don’t appear to have ovulated this month, you may still have. It’s possible to miss the LH surge just by chance. This is more likely if you don’t test at the same time each day, or you drink a lot of water.

Remember too that ovulation tests are imperfect, and it’s possible to get a false negative. If you had other symptoms of ovulation, such as pain, or changes in your mucus (see Changes during the menstrual cycle), it’s likely that you ovulated anyway. However if you have gone two or three months with consistently negative tests, then you might not be ovulating regularly. In that case, it’s worth seeking medical advice.

Healthy conception

When you’re trying to conceive, you’ll find you are much more aware of your general health. As a rule, colds, flu, and other common infections are unlikely to affect your fertility or your unborn baby if you have conceived. Some infections and viruses, however, can have a more serious impact:

  • Shingles and chickenpox (both caused by the same virus) are best avoided around the time of conception if you haven’t had chickenpox before.

  • Food poisoning, for example caused by listeria bacteria, can be harmful .

  • Toxoplasmosis can be contracted from handling cat feces (see Protect yourself).

Too much testosterone can affect a woman’s fertility.

Small quantities of testosterone are secreted from the adrenal gland and the ovaries. In low levels it may aid fertility, but too much can affect the menstrual cycle and lead to infertility.

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