Hurrah! One lucky sperm has successfully managed to penetrate into the innermost part of the egg (oocyte). So fertilization has taken place and you’ve conceived and are officially pregnant now! The single cell which is formed as a result of fertilization is now called a zygote. It quickly forms a thick wall around itself, to prevent other sperms from penetrating.

Read: Pregnancy after 35

Twins are decided at this stage

At three weeks pregnant, embryos form. So if two or more eggs happen to be released and fertilised, you will have non-identical twins.  But if a single fertilised egg splits into two zygotes, two separate embryos are formed which will go on to become identical twins.

Read: Signs of a twin pregnancy

Your baby at Week 

• The single celled zygote is now a blastocyst. Blastocyst is nothing but a mass of cells that are rapidly multiplying.
• Meanwhile, the part which is to develop into the placenta begins to manufacture the pregnancy hormone – HCG (human chorionic gonadotropin). This hormone sends a message to the ovaries to stop releasing eggs and also triggers your body to produce more of the pregnancy hormones – oestrogen and progesterone. These hormones prevent the uterine wall from shedding besides nourishing and stimulating the growth of the placenta.
• At the same time, the amniotic fluid is filling up around the ball of cells. This will go on to become the amniotic sac containing the amniotic fluid which will protect your baby in the coming weeks and months.

Read: Is it safe to exercise during pregnancy?

Pregnancy symptoms at 3 weeks

• At this stage, there is little chance of you knowing you are pregnant unless you have a strong suspicion. This is also the time when your body may be showing early pregnancy symptoms – a few of which may be similar to the pre-menstrual symptoms (stomach cramps and fatigue) you experience.
• Many women may have implantation bleeding or spotting a few days after implantation.
• A slight rise in body temperature,  headache and decreased consistency of vaginal discharge are other symptoms

Read: A guide to using home pregnancy test kits

Is it a good time to do a pregnancy test?

If you’re having a 25-28 cycle, this is probably the time you’re expecting your period. But if you’ve been actively engaged in the baby- making process, you may be eager to do to a pregnancy test. At this stage, there is a 50/50 chance of your pregnancy test showing positive. Most home tests can accurately detect pregnancy in the week after your period is due.