This is one of the most crucial stages of pregnancy as this is the week when you conceive . Many women do not know they are pregnant until week 3 as there are often no visible signs of pregnancy. Pay a visit to your doctor to find out what you can expect from your pregnancy.
Changes in Your Body during Pregnancy Week 3
Your body has just started producing estrogen and progesterone, the hormones responsible for most bodily changes during pregnancy . However, no detectable changes, such as increased belly size, occur at this point.
The hCG (Human Chorionic Gonadotropin) Levels
The placental cells start producing hCG (pregnancy hormone) immediately after implantation . The hCG level then continues to increase until around 8-12 weeks, when it stabilizes or drops. The higher the hCG levels, the more evident the symptoms .
Development of Your Baby during Week 3 Pregnancy
The matured egg that travels down the fallopian tube gets fertilized by a single sperm out of around 300 million . This successful sperm has to travel 6-8 inches up the uterus before it can reach the egg waiting for fertilization. It takes around 20 minutes to penetrate the hard outer layer of the egg and within the next eleven hours, the nuclei of the sperm and the egg merges with each other, completing the fertilization process .
The egg and the sperm contain 23 chromosomes each, which then combine to form the zygote containing 46 chromosomes . The gender of the baby is decided the moment the egg gets fertilized, depending on whether the sperm fertilizing it carries an X chromosome or a Y chromosome. Other prominent chromosomal traits such as hair color, eye color and body type are also determined at this stage .
The newly formed zygote travels up the fallopian tube towards the uterus where it gets attached to the uterine wall for further development (implantation) . The implantation process completes 6 to 12 days (9 days in average) after ovulation and fertilization . In the meantime, the zygote continues dividing to form a cluster of around 100 cells (blastocyst) . In some rare cases (about 2% of pregnancies), the blastocyst attaches itself somewhere in the fallopian tube, ovary, abdomen or cervix, leading to an ectopic pregnancy .
Signs and Symptoms at Pregnancy Week 3
Most women experience the implantation spotting or bleeding within 1-2 days after implantation . Additional signs may include:
- Increased basal body temperature 
- Lower abdominal cramping
- Decreased consistency of cervical mucus or vaginal discharge 
Other early signs of pregnancy, such as nausea (morning sickness), breast tenderness or soreness, food cravings , fatigue, gas, bloating, mood swings , lower back pain and diarrhea (rare), may also be present.
Tests and Exams for Pregnancy Week 3
Most pregnancy tests detect conception based on the hCG levels in the urine. So, conception can be detected by a home pregnancy test during the third week , 5-6 days following implantation. Blood pregnancy tests give positive results 3-4 days after implantation . At week 3, the baby is just a tiny ball of cells which cannot be detected on scanning exams like ultrasound.
Tips for a Healthy Pregnancy
Most women do not find out they are pregnant before the third week. So, this is the time to make all the dietary and lifestyle changes necessary for a healthy pregnancy and baby.
- Giving up alcohol and tobacco
- Cutting down your caffeine intake
- Making sure to inform the health care practitioner about your pregnancy before taking an x-ray for any health issues (e.g. dental ailments) as x-ray is often not recommended during early pregnancy 
- Avoiding going near hazardous substances such as pesticides (it may hamper the cell division)
- Taking pre-natal vitamins containing folic acid (0.4 milligrams daily) as it prevents most birth defects involving the brain and spinal cord most likely to occur during the first few weeks 
- http://www.webmd.com/baby/folic-acid-and-pregnancy [/ref]