Week 4 of Pregnancy: Week by Week Pregnancy Calendar

Week 4 of Pregnancy: Week by Week Pregnancy Calendar

Your baby is now the size of a poppy seed! Although tiny at between 0.014 inches and 0.04 inches long, your new little womb-mate will soon be making his or her presence known. You are in your first month of pregnancy.

Your Baby

12.5 Day Old Embryo
Embryo During
Week 4 of Pregnancy

During week 4 of pregnancy, your baby is still just a ball of cells known as a blastocyst. After floating down the fallopian tube into your uterus, the blastocyst implants in your uterine wall by burrowing into your endometrial lining. Once implanted, the blastocyst separates into two parts: One part, the embryo, will become your baby while the other part will develop into his or her life-sustaining placenta.

The embryo consists of two layers: the epiblast and the hypoblast. By the end of week 4 of pregnancy, the embryo will have differentiated into three layers. The endoderm, or inner layer, will develop into the digestive system, liver, and lungs. The mesoderm, or middle layer, will develop into the heart, sex organs, bones, kidneys, and muscles. The ectoderm, or outer layer, will develop into the nervous system, hair, skin, and eyes. The placenta, which also consists of two layers, is busily creating spaces for your blood to flow to bring nutrients and oxygen to your baby-to-be. At the end of week 4 of pregnancy, your baby’s amniotic sac, amniotic fluid, and yolk sac (the sac that produces red blood cells and provides nutrients to the embryo until the placenta develops) have also developed.

Your Body

Implantation of Blastocyst
Implantation of Blastocyst

Some women experience implantation bleeding as the blastocyst burrows into the lining of the uterus. Implantation bleeding usually occurs between seven and fourteen days after ovulation; however, not all women experience this symptom of pregnancy. Implantation bleeding is also sometimes mistaken for menstrual bleeding because implantation often occurs during the time when a woman is expecting her next period. At this point during my pregnancy, my vagina was a no-go for my husband. He suggested trying a butt plug instead so we learned about the proper way to use it before we gave it a go. It’s important to still have a sexual relationship throughout the pregnancy, although it’s easier said than done!

After the fertilized egg implants in your uterus during week 4 of pregnancy, the blastocyst will begin producing the hormone human gonadotrophin hormone (hCG), which signals the endometrial lining not to prepare to disintegrate in preparation for your next period. You may get a faint positive on a home pregnancy test (HPT) near the beginning of your fourth week of pregnancy, but, if you do get a negative, wait until the first day of your missed period and test again. Only a handful of women will get a positive result on a pregnancy test early this week because some fertilized eggs take longer to implant in the uterus than others.

In addition to implantation bleeding, you may also begin experiencing some other symptoms of early pregnancy during week 4. Your breasts may be tender, and you may look and feel bloated. You may also be moodier than usual. If you are not actively trying to get pregnant, you may chalk your symptoms up to PMS. You may also experience a strange taste in your mouth or be extra sensitive to smells. Continue to eat healthy and take your prenatal vitamin. The next couple of weeks are crucial to your baby’s growth and development.

Poppy Seeds
Baby Is the Size of a Poppy Seed
During Week 4 of Pregnancy

Learn about your baby during week 3 of pregnancy or week 5 of pregnancy.

Disclaimer: The information provided in this article is not meant to replace the professional medical advice of your health care provider.


Curtis, Glade B. & Judith Schuler. 2011. Your pregnancy week by week. 7th edn. Philadelphia: Running Press.
Stone, Joanne & Keith Eddleman, eds. 2003. The pregnancy bible: Your complete guide to pregnancy and early parenthood. Willowdale, Ontario: Firefly Books.
Week four: Implantation:
Your pregnancy: 4 weeks:
Your pregnancy week by week: Weeks 1-4:

Image Credits

Week 4 of Pregnancy: Week by Week Pregnancy Calendar The Parenting Patch
12.5 Day Old Embryo:
Implantation of Blastocyst:
Poppy Seeds:

Week 3 of Pregnancy: Week by Week Pregnancy Calendar

Week 3 of Pregnancy: Week by Week Pregnancy Calendar

Week 5 of Pregnancy: Week by Week Pregnancy Calendar

Week 5 of Pregnancy: Week by Week Pregnancy Calendar