3 Weeks Pregnant

Congratulations! The third week of pregnancy is usually when ovulation and implantation occur. Here’s what you should know about your body and your baby this week.

How is the Third Week of Pregnancy Calculated?

There is oftentimes a lot of confusion when discussing the way in which pregnancy is calculated. Because most women do not know when they conceived (as it is difficult to know exactly when ovulation occurs), pregnancy is always determined from the first day of your last menstrual period (LMP). Given that this is the way pregnancy is calculated, your pregnancy will last around 40 weeks. For more information on how pregnancy is measured, please see our information on calculating your dates.

What changes are occurring with your body when you are 3 weeks pregnant?

Many women do not notice any changes, but some women experience mild cramping and increased vaginal discharge around ovulation. During ovulation, the ovaries release a mature egg that begins to travel through the fallopian tubes towards the uterus. Sperm travels through the uterus to fertilize the egg within the fallopian tube. Only a single sperm can fertilize an egg. Both the sperm and the egg contain 23 chromosomes that will combine to make up the zygote which contains a total of 46 chromosomes. At conception, your baby’s gender, eye color, hair color, and many other features have already been determined. The zygote continues to travel through the fallopian tubes into the uterus where it will attach itself to the uterine wall. For additional information on ovulation please read Ovulation Frequently Asked Questions or Understanding Ovulation.*

What is happening with your baby at 3 weeks pregnant?

The blastocyst is growing and multiplying rapidly at this point. Part of the blastocyst will become the placenta, which will produce hCG (human chorionic gonadotropin). This is the pregnancy hormone that is detected by pregnancy tests. Amniotic fluid is also collecting around the cells and will protect and cushion your baby throughout your pregnancy. Significant aspects of fetal development begin during this week, including the development of the brain, spinal cord, heart, and gastrointestinal tract.*

How big is your baby when you are three weeks pregnant?

The embryo in your body is very small and actually looks more like a group of cells than a baby. It is the size of the head of a pin and approximately .0019 inches (.048 mm) long. It could be seen by the human eye if it were not inside your body.*

What should you plan for this week?

Exercise is important during pregnancy. Consult your healthcare provider before beginning or continuing an exercise routine. In most circumstances, if you are already on an exercise routine, then you can likely continue your active lifestyle. Remember that your baby is taking in everything that you do, both good and bad. You want to avoid alcohol, illegal drugs, certain medications, certain foods, caffeine, and smoking. Nutrition is also important this early in pregnancy. Folic acid and other essential nutrients and vitamins are necessary for fetal growth and healthy pregnancy. Eating a balanced diet with adequate amounts of protein and calcium is also important for both you and the baby. If you have questions, check with your health care provider to see what dietary recommendations they encourage.

Tips for making your pregnancy better

Your body is beginning to go through some drastic changes, and you may feel overwhelmed at times. Make sure that you do not neglect to get adequate rest and nutrition. Take time to relax and enjoy being pregnant. Stress is the worst thing for your body and baby during pregnancy, use techniques to keep your stress levels low.

Tips for mom’s partner

Both you and your partner may have fears that begin to emerge, even if this was a planned pregnancy. Be open with each other, and discuss your concerns. Ask questions so she feels you want to be involved in the pregnancy. Take time to plan little ways that you can offer support and surprise her over the next 37 weeks.

Want to Know More?


Mayo Clinic

Centers for Disease Control