3 weeks pregnant: Changes for mum

While you don’t know you’re pregnant and your body appears the same to the naked eye, there are enormous changes occurring inside you. Your ovaries have just released a mature oocyte (egg) for fertilisation in the fallopian tube (usually day 10-14, in week 2). Assuming ovulation occurs on day 14, fertilisation is likely to occur on day 15, that is the first day of pregnancy week 3. If ovulation occurred a day or two earlier (e.g. day 13) or later (e.g. day 15), the egg would also have been fertilised a day or two earlier (e.g. day 14) or later (e.g. day 16).

For fertilisation to occur, your egg needs to come into contact with your partner’s sperm. The outer layer of your egg, called the corona radiata, facilitates this meeting by releasing chemicals to attract sperm. It needs to attract just one of the 40 million or so sperm which are released with each ejaculation for fertilisation to occur.

When a sperm comes in to contact with your egg it releases its own chemicals from a cap in the sperm head called the acrosome. The sperm’s chemicals help it to penetrate the egg’s shell, a gel like layer called the zona pelucida. Usually more than one sperm will reach the egg and attempt to penetrate it, but chemicals released by the egg and the successful sperm ensure that only one ever manages to get inside the shell. Once a single sperm has gone through the shell, the outer layer of the egg changes so that it locks out the other sperm. The single successful sperm enters the middle of the egg, and, hey presto! Your baby’s development has begun.

Once the sperm reaches the middle of the egg, the genetic information contained in the egg and sperm, known as chromosomes, fuse together. Both the egg and sperm have 23 chromosomes each. When these chromosomes fuse together they form a single 46 chromosome cell from which your baby grows. All cells in the body except the sperm and egg have 46 chromosomes. Chromosomes provide the information for generating new cells and are responsible for determining everything from the sex of your child, to the colour of its eyes and hair, its personality and level of intelligence, and even whether the child will be predisposed to certain health conditions (for example, diseases that run in your family or your partner’s family).

References

More information on the 3^rd week of pregnancy

For more information on the third week of pregnancy, see 3 weeks pregnant.

For more information about what happens to create your growing baby after conception, see Changes for baby this week.

For more information about preparing your body for pregnancy with healthy eating and lifestyle changes, see Pregnancy health information this week.

For more information about the key points on the third week of pregnancy, see Key points this week.