Weekly Guide to Pregnancy: Week 4

Week 4
Week 4 

Your Pregnancy

How do I know how pregnant I am, and how is my pregnancy measured and dated?

A woman's menstrual cycle is on average 28 days long. Using this 28-day average, fertilisation or conception normally occurs at around day 14 or the middle of your cycle (2 weeks after your last period and 2 weeks before your next period). Although (based on an average 28-day cycle) conception is likely to occur on day 14 of your cycle; the start of your pregnancy is actually calculated from the start date of your last menstrual period (LMP), or day 1 of your cycle.

Given this method of calculation, your pregnancy is measured in what is termed gestational weeks, as opposed to being measured from the actual day of conception. There are 40 gestational weeks of pregnancy. Assuming that you have a 28-day cycle, in gestational weeks 1 and 2, your baby is waiting to be conceived. On day 14,and at the start of gestational week 3, your baby has just been created. By the end of gestational week 3, the actual age of your baby is one week.

What if I don't have a regular 28-day cycle and ovulation doesn't usually occur on day 14?

If your menstrual cycle is longer than 28 days, or irregular, the day of conception could be any time after day 14, and it will be more difficult to confirm exact dates in the early stages of your pregnancy. To determine the gestational age of your baby, your doctor may perform a blood test to measure the level of pregnancy hormone, or you may undergo an early ultrasound to confirm exact dates.

What actually happened when my baby was conceived?

At the moment of conception, one of your partner's sperm entered and fertilised your egg, which then seals itself off to exclude all other sperm. An egg provides 23 chromosomes from the mother, while the father's sperm provides the other 23 chromosomes. Together they make up a full compliment of 46 chromosomes; the full genetic make-up of cells needed to create a human being.

How and when is the sex of my baby decided?

The male sperm provides the sex-identifying chromosome, so it is the man, and the type of sperm (male or female) that fertilises the egg, that unwittingly decides the sex of your baby. A sperm bearing the Y chromosome will produce a boy, while a sperm carrying the X chromosome will result in a girl.

What happens immediately after fertilisation?

When your egg is fertilised by your partner's sperm, the genes or chromosomes from each of you combine to create a cell. This cell then starts to divide, becoming a collection of cells, or blastocyst.

The blastocyst continues to divide and grow, moving down the fallopian tube until it reaches the uterus (or womb) between 3 and 7 days later. Once in the uterus, the blastocyst will implant itself into the lining of the uterus, but before this happens, a change occurs. The blastocyst draws in liquid, creating a liquid pocket in its centre. This fluid space creates a division in the blastocyst, resulting in an inner cell mass, which will form the embryo, and an outer trophoblast, which will form the placenta.

What is implantation and when does it happen?

About a week after conception, the outer cells help the blastocyst to implant or embed into the lining of the uterus so that it can seek nourishment. As a result of the hormone changes following conception, the lining of the uterus has already become thicker and has an increased blood supply in preparation for implantation. While the blastocyst implants deeper into your uterus, the vascular network and placenta begin to establish themselves. Until the they are ready to nourish the blastocyst, it will feed off the lining of your uterus.

How big is my baby and what is it called?

When the blastocyst implants in the lining of your womb, it has grown to about 0.23mm (0.009 inches) long.

For the first 8 weeks of your baby's development (10 weeks of gestation), your baby is called an embryo. After this time and until the birth, the developing baby is called a foetus.

Will I even know I'm pregnant?

The majority of women are not aware of any of these developments in their body, as they have not even missed a period, which is one of the first probable signs of pregnancy. If you are consciously trying to conceive, you may be more aware of what is happening with your body, and you will probably perform a home pregnancy test as soon as your period is late.

What should I be doing if I'm trying to conceive?

While you probably won't know that you are pregnant yet, here are some tips for weeks 2-4:

  • Start taking folic acid (it's best to start taking folic acid 3 months prior to conception if possible)
  • Eat a healthy, balanced diet
  • Quit smoking
  • Eliminate alcohol
  • Try to limit caffeine intake to 1 cup of coffee per day
  • Incorporate a moderate exercise plan into your day

Learn more about your pregnancy

Read more information on Pregnancy and Birth on Essential Baby.

Discuss Pregnancy in the Essential Baby Forums.

The First Trimester

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The Second Trimester

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The Third Trimester

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