What is Implantation?

Implantation is the process where a fertilized egg, also known as an embryo, attaches to the uterine wall by penetrating the first the lining, also known as the endometrium. This allows the embryo to connect with the blood circulatory system of the mother to form the placenta, which becomes the nutrient filter between the mother and fetus. A normal pregnancy is impossible without successful implantation.

When conception happens through intercourse, the fertilization of the egg with the sperm occurs in the outer portion of the fallopian tube. The fertilized egg then stays in the tube for a time period of a few hours, during which it grows further and becomes capable of implantation. At this time the uterine lining also undergoes changes to become receptive to the embryo. The embryo travels towards the uterine cavity and arrives around 3 days after ovulation. At this stage, the embryo is small and survives on endometrial secretions and dumping its metabolic wastes into the lumen of the uterus.

The embryo is continuously growing and hatches out of the hard shell around it called the zona pellucida, after which it finds a spot within the uterine cavity on the lining which is appropriate for it to latch on to. Once it has selected a spot it attaches and buries itself into the uterine lining until it has implanted just deep enough to find a connection with the mother's blood supply and start the developing placental connection.

Signs of Implantation

The “Implantation Window”- When Does Implantation Occur?

The “implantation window” refers to the time frame from when the uterine lining is receptive to the free-floating embryo and when the embryo is also ready to take hold, leading to a successful implantation. Implantation does not generally occur outside of this window. Studies have shown this window to span from the 6th to the 10th day after ovulation.

The process of implantation begins in the first week of development, 5-7 days after ovulation or fertilization and 2-3 days after the fertilized egg enters the uterus. This corresponds to day 18 or 19 of a normal menstrual cycle.

In in vitro fertilization (IVF) cycles, if the embryo has been transferred on day-3 of development, it grows further until it hatches and implants around 3-4 days after embryo transfer. And if a day-5 embryo (blastocyst) has been transferred, it would implant 1-2 days after the transfer to the uterine cavity.

Factors Contributing to a Successful Implantation

The two main factors contributing to a successful implantation are a healthy growing embryo and a receptive uterine lining. While the list isn't long, the two items are integral to successful implantation.

What Can Cause Implantation Failure?

Decreased endometrial receptivity due to uterine cavity abnormalities like fibroids, scarring and adhesions or Mullerian defects like uterine septa, thin endometrium and some medical conditions in the mother like circulatory disorders and immunological factors, have been studied as the reasons for implantation failure. In women with certain gynecologic disorders, including endometriosis, tubal disease, and polycystic ovary syndrome, endometrial receptivity seems to be compromised, leading to infertility and pregnancy loss.

Is There a Way to Know the Cause of Implantation Failure?

Recurrent implantation failure (RIF) in natural intercourse cycles will present as infertility, where pregnancy fails to achieve. In IVF cycles RIF is determined when transferred embryos fail to implant after several IVF treatment attempts. In such cases, the cause can be investigated in various ways depending on the clinical details of the people involved. This may include:

  • Genetic testing for conditions that can be transferred to the child
  • The uterine cavity and its lining can be assessed
  • The mother can be tested for various blood and immune disorders
  • Various assessments of the embryo with genetic testing sometimes can be done

Increasing the Odds of Implantation

If a cause for RIF is detected, it has to be medically corrected. Otherwise, there is no other definitely proven way to improve implantation. However, some methods have been used empirically especially in IVF treatments. These include hormonal medication like estrogen, and medicines like vitamin E, baby aspirin and Sildenafil in an attempt to improve the blood supply to the uterus. Certain procedures used for the same purpose include "scratching" the uterus in the cycle previous to the embryo transfer; and assisted hatching which is a procedure in which the outer covering of the embryo is partially opened to facilitate embryo implantation. Dietary supplements or other methods like acupuncture have not yet been proven to have any significant effect on implantation.

What are the Signs of Implantation?

The most accurate way of knowing if you are pregnant is a pregnancy test, which detects the rise in human chronic gonadotropin (hCG) a hormone found in a pregnant woman’s blood or urine. These tests are generally most accurate on or after the day of the next expected menstrual period. This delay is also known as the two week wait (2WW), the time period between potential conception and the ability to confirm a pregnancy. However, there are a few signs and symptoms that may hint towards changes occurring due to implantation and early pregnancy. None of them, however, are very specific or diagnostic of pregnancy. The most common of these are a missed period, breast tenderness, increased urination, nausea and unexplained fatigue. Other less obvious signs and symptoms of pregnancy are light spotting, cramping, moodiness, bloating and constipation, nasal congestion and food aversions.

  • If in a woman in her reproductive years, with generally regular menstrual cycles, a week or more passes without the start of an expected menstrual cycle, she might be pregnant.
  • Hormonal changes of early pregnancy could cause sensitive and sore breasts. The discomfort generally decreases as pregnancy advances.
  • There could also be increased urination. The volume of blood increases during the first trimester of pregnancy, causing the kidneys to process and excrete extra fluid.
  • Morning sickness of pregnancy is a variable symptom occurring due to the rising hormones of pregnancy. It often begins a month after implantation. However, some women feel it earlier and some never experience it.
  • During early pregnancy, the rapidly rising levels of the hormone progesterone make one feel sleepy and fatigued.
  • Sometimes around the time of an expected menstrual period, a small amount of light spotting could be indicative of pregnancy. Known as implantation bleeding, it happens when the fertilized egg attaches to the lining of the uterus, and is seen externally only about 10 to 14 days after fertilization. However, only 1 out of 3 pregnant women experience it. Moreover, there are other causes of bleeding in early pregnancy like abnormal placenta formation, an ectopic pregnancy that has implanted outside the uterine cavity, and others.
  • Some women experience mild uterine cramping early in pregnancy.
  • The flood of hormones in early pregnancy can make a pregnant woman unusually emotional with mood swings and crying episodes. They can also cause a bloated feeling. The rising progesterone slows down the digestive system and can lead to constipation. Increasing hormone levels and blood production can cause the mucous membranes in the nose to swell causing a stuffy or runny nose. Some women become more sensitive to certain odors and the sense of taste might also change.

Unfortunately, many of these signs and symptoms aren't unique to pregnancy. They could be indicative of another illness or be premenstrual symptoms which are very similar. Likewise, one can be pregnant without experiencing any or most of these symptoms. Therefore, they are not considered as reliable signs of pregnancy since they are not very specific.

You may choose to record any of the above symptoms and correlate them with the diagnosis in retrospect, but should not to rely on them as indicators of pregnancy.