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zygote dividing in two

What's happening inside

When the two nuclei have pooled their genetic material to create a cell containing its full complement of 46 chromosomes-23 each from the mother and father-the cell can start dividing, shown here at the first division creating a two-celled body.

Vital cell divisions are now taking place as the fertilized egg begins its journey toward implantation.

The chromosomes from the sperm and egg joined over 24 hours ago. It takes around 30 hours for the resulting zygote to complete its first cell division. The zygote, at only 0.1 mm in diameter goes on to divide into 16 cells, forming a compact ball.

Cell division is such that the ball of cells is hardly any larger than the original zygote. The ball of 16 cells, now known as a "morula" (as it resembles a mulberry), travels toward the uterus, entering on day three after fertilization. Every cell within the morula is totipotent, meaning it is able to form any type of cell. From this point onward the cells will lose this function as they start to specialize.

Ask A... Doctor

I've been doing ovulation tests. I've now ovulated so do my partner and I need to keep having sex to make sure I conceive?

You can't be sure that you've conceived already, so the usual advice would be to continue lovemaking. Even if you've been tracking your ovulation by monitoring your temperature or cervical mucus, or by using an ovulation kit (see Are you ovulating?), you can't be sure exactly when it occurred. It won't be possible for you to pinpoint the exact time of ovulation.

Since the fertile window is several days, you may as well continue having sex for at least a couple of days after what you think is your most fertile time.

Furthermore, since sex says "I love you" more strongly than most other means of communication, it's good for both you and your partner to stay intimate at times you're not trying to conceive.

Remember also that abstention doesn't usually have the hoped-for effect of banking up and improving the quantity and quality of sperm. In fact, the opposite may happen (see This is Day 13 of your Menstrual Cycle).

Focus On... IVF

From eggs to embryos

Egg collection will be scheduled, following the first stage of IVF. Not all follicles that were stimulated will contain eggs. Two days after egg retrieval, you will be given progesterone to thicken the uterus lining. Two to five days after fertilization, the most promising embryos are chosen to be transferred.

If you're under 40, you'll have one or two embryos transferred. If you're over 40, you may have up to three or more transferred. The goal is to achieve a pregnancy, yet limit the risks of a multiple pregnancy. Any leftover embryos can be frozen for future treatment cycles. Recent research suggests that frozen embryos are better than fresh ones-this may be because only the best embryos are selected for freezing and survive the freezing and thawing process.

The outcome of IVF depends to a great extent on the woman's age, but on average each cycle has a 20 percent success rate.

woman having eggs collected for IVF

excerpted from:

Excerpted from Pregnancy Day by Day.
Copyright © 2008 Dorling Kindersley Limited.
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