After the long 6-day trip from the fallopian tubes, the blastocyst, which has since divided into two sections, arrives within the uterus and begins to burrow or implant into the wall of the uterus.1-3
At this point, the blastocyst is a grouping of cells comprised of an inner collection of cells that will ultimately develop into the embryo, and an outer shell that provides protection and nourishment to the growing embryo.1,3
During this time, the amniotic sac is forming, as is the yolk sac and the placenta.1,3
This MNT Knowledge Center feature is part of a series of articles on pregnancy. It provides a summary of each stage of pregnancy, what to expect, and insights into how your baby is developing. Take a look at the other articles in the series:
You will also see introductions at the end of some sections to any recent developments that have been covered by MNT's news stories. Also look out for links to information about related conditions.
Symptoms at 4 weeks pregnant
Due to the stage of the menstrual cycle, the uterus is thick with blood providing the perfect environment to support the growing embryo.2
During week 4 of pregnancy, the fetus is growing rapidly and its development is composed of three layers - the ectoderm, mesoderm and endoderm.
Approximately 6-12 days following conception, some people experience mild vaginal spotting with or without mild cramping. This is referred to as implantation bleeding and may be mistaken for a menstrual period.4
Generally, this bleeding will resolve on its own. Implantation bleeding is an early sign of pregnancy. However, if the bleeding is heavy, you are experiencing chills, fever or cramps, contact your health care provider.4
Bleeding during pregnancy has causes other than implantation and may be caused by sexual activity, an ectopic pregnancy or miscarriage.4
Some types of sexual activity can cause bleeding at any time during your pregnancy likely due to hormonal changes. If an ectopic pregnancy has occurred, this means that the embryo has implanted outside the uterus and is considered a medical emergency.4
Medical attention should be sought immediately if you are experiencing bleeding with pain and cramps.4
Miscarriage is a common occurrence, with approximately 15% of pregnancies ending in the loss of the fetus. Call your medical provider if you have bleeding and cramping or think you may be experiencing a miscarriage.4
At this stage of development, you may start to have some symptoms of pregnancy including breast tenderness, headaches, low backaches and nausea.5
Now may be a good time to take a pregnancy test if you are late for your period, as this is the earliest possible time that a home pregnancy test will be able to detect a pregnancy.5
Fetal development at 4 weeks pregnant
The fetus is growing rapidly and its development is composed of three layers, the ectoderm, mesoderm and endoderm, which will eventually form the baby's organs and tissues.3,5
At 4 weeks pregnant, the fetus is the size of a poppy seed.
The nervous system and brain, epidermis (outermost layer of the skin), hair, lens of the eye, pigment cells, nails, mammary glands, sweat glands and tooth enamel will develop from the ectoderm layer.3,5
The heart, lymph cells, sex organs, skeleton and skeletal muscles, connective tissues, urogenital system, dermis of the skin, the kidneys and spleen will develop from the mesoderm layer.
The endoderm layer will develop into the lungs, liver, pancreas, pharynx, stomach, urinary bladder, parathyroid, intestines and thyroid and the lining of the urethra.3,5
Fetal size at 4 weeks pregnant
Right now, the fetus is still quite tiny, the size of a poppy seed, measuring only approximately 0.078 inches.3,5
Recent developments on implantation from MNT news
It is well established that consuming alcohol during pregnancy can cause harm to the fetus. Now, a new study finds that drinking alcohol as early as 3-4 weeks into pregnancy - before many women even realize they are expecting - may alter gene functioning in the brains of offspring, leading to long-term changes in brain structure.