In order for implantation to take place, ovulation and fertilization must occur first. Implantation happens early in the pregnancy stage after the fertilized egg (zygote) travels down the fallopian tube to the uterus and adheres to the epithelium, or uterine lining. It takes about eight to 10 days for the fertilized egg to travel to the uterus. During that time, it develops into a blastocyst through several stages of transformation—beginning as a single cell that divides into about 150 cells, with an outer layer (the trophoblast), a fluid-filled cavity (the blastocoel), and a cluster of cells on the interior (the inner cell mass). This tiny ball of cells is about the size of a poppy seed. It adheres to the epithelium during week 4 of gestation. Once it is firmly attached, it is called an embryo. The embryo will then divide into two parts that will become the placenta and the fetus—your future boy or girl.
An ultrasound examination done during weeks 5 to 6 of gestation may show the amniotic sac (or bag of waters) and yolk sac, which are forming at this time. The amniotic sac is where your baby will develop. The yolk sac will later be incorporated into your baby’s digestive tract. This ultrasound finding confirms that implantation has taken place and that your road to parenthood has begun.
Implantation and Pregnancy Testing
When implantation takes place, the cells that form the placenta produce the hormone human chorionic gonadotropin (hCG). In a healthy pregnancy, the level of hCG will rise rapidly and can be detected in your urine by a home pregnancy test 8 to 14 days after conception (fertilization) and in your blood by a test done at your practitioner’s office as early as 6 days after implantation—before your first missed menstrual period.
The function of hCG is to “tell” the corpus luteum to continue producing progesterone, which is necessary for the successful maintenance of pregnancy. Because of its important role in the development of the fetus, the hCG level is an excellent indicator of pregnancy. When using a home pregnancy test kit, keep in mind that its accuracy is greatly affected by the timing of when the test is taken, the test’s sensitivity, and correct usage. Also, hCG levels can vary tremendously among women. Taking the test too early could lead to a negative result if your hCG level is too low to be detected by the test you used. On the other hand, an early test could show a positive result, but a later test may be negative. This happens in a chemical pregnancy, in which the fertilized egg begins to implant in the uterus, but for some reason it does not complete implantation. Many chemical pregnancies pass unnoticed as a late period. To ensure the most reliable result, perform a home pregnancy test one to two weeks after you miss your period.
You may be one of the few women who experience staining or spotting when implantation happens. “Implantation bleeding,” as it is called, may occur 10 to 14 days after fertilization and is normal and temporary. The attachment of the fertilized egg to the uterine lining is believed to cause the bleeding. It is possible to mistake implantation bleeding for your period, because both take place around the same time of your menstrual cycle. However, implantation bleeding is typically of a shorter duration and much lighter than menstrual bleeding.
If you had a lighter than normal period along with the more common early symptoms of pregnancy, including breast changes, bloating, fatigue, frequent urination, and nausea (read about these symptoms by following the links), you should consider taking a home pregnancy test. If you are pregnant, the earlier you begin prenatal care, the better. Vaginal bleeding that persists or that is accompanied by unusual pain or cramping could be sign of an abnormal implantation. This warrants immediate medical attention.
Why Implantation Succeeds
One of the mysteries of conception has been why the mother’s body does not reject the fertilized egg from implanting. Technically, the blastocyst is a group of foreign cells in the body and a natural response of the immune system is to seek out and destroy such cells. Several mechanisms are thought to prevent your body from rejecting implantation:
Maternal immune cells are silenced – The maternal immune cells are silenced or disabled by chemokine secretion and by corticotropin-releasing hormone secretion by the mother and the blastocyst.
Decidualization – There is a decidual reaction that takes place after implantation; the decidua is the part of the mucous membrane lining the uterus that undergoes special modifications in preparation for and during pregnancy. This reaction is promoted by the mother’s hormones, estrogen and progesterone, and creates a friendly environment for the embryo to develop.
Cervical mucus plug – Estrogen stimulates production of a cervical mucus plug from glands at the opening of the uterus, the cervix, where it joins the vagina. This plug acts in a mechanical and antibacterial manner, protecting the interior of the uterus.
Abnormal implantation of a fertilized egg often leads to pregnancy loss. Fortunately, this is a rare occurrence and most women experience normal pregnancies. An implantation is considered abnormal in the following circumstances:
Ectopic pregnancy – In an ectopic pregnancy, the blastocyst implants outside the uterus. This most commonly occurs in the fallopian tube but can also occur in the cervix, on the ovary, or in the abdomen. An ectopic pregnancy has to be removed, either surgically or with medication, to avoid rupture of surrounding structures, which can result in severe bleeding and shock. Rupture could greatly reduce the chances of conceiving in the future and can be life-threatening to the mother.
Hydatidiform mole – A hydatidiform mole may occur when a blastocyst implants but no embryo develops. Instead, a tumor arises in its place. The mother may experience symptoms of pregnancy and may test positive on a home pregnancy test because the cells in the tumor will produce hCG. An ultrasound examination that shows the absence of an embryo confirms the diagnosis of hydatidiform mole.
Problems with the placenta – A variety of placental abnormalities can occur depending on the site of implantation. If any of the functions of the placenta are impaired, a pregnancy may not be able to continue to full term. Problems with the placenta are a common cause of pregnancy loss. When they occur early in pregnancy, the result is often a miscarriage. Later in pregnancy, a placenta problem results in pre-term delivery. However, it could cause complications, or death, in both the mother and baby.