Expert / 18 June, 2021 / Mr Raef Faris
Implantation is when a fertilised egg attaches to the lining of the uterus marking the beginning of pregnancy. It is a highly complex process that involves an interaction between the embryo and the lining of the uterus.
The woman’s body goes through various preparatory steps behind the scenes before the urine or blood tests indicates positive pregnancy. The main changes occur in the ovaries and womb. The ovary starts growing a follicle which usually contains an egg and also produces oestrogen hormone. The hormonal changes support the uterus which goes through structural and functional remodeling, preparing the lining to receive the fertilized egg.
The optimal time of intercourse would be five days before ovulation and up to 24 hours after ovulation. The ejaculated sperm after intercourse travels from the vagina through the neck of the uterus into the cavity of the womb and then enters the fallopian tube within hours after sex.
The egg is released from the ovary around the middle of the menstrual cycle. However, this can vary and in some cases, the egg can be released much earlier or much later in the menstrual cycle. After ovulation, the egg gets picked by the fallopian tube. The egg meets the sperm in the fallopian tube where fertilisation usually takes place.
After ovulation, the remnants of the follicle in the ovary starts releasing progesterone hormone. This hormone plays a crucial role in implantation, preparing the lining to receive and support the development of the fertilized egg (embryo)
The fertilised egg (embryo) then travels down the Fallopian tube and enters the cavity of the uterus. During this period the cells within the embryo starts dividing quickly to prepare itself to implant. The fertilized egg starts as one cell containing the genetic material of both the egg and the sperm. On day two of the embryo’s life, it divides into two to four cells, on day three it becomes five to eight cells. Around day four of the embryo’s life, it enters the cavity of the uterus.
This stage is called “morula stage” where the embryo cells are not clearly distinguished. The embryo spends approximately 72 hours in the cavity of the womb before implantation. On days five to seven of the embryo’s life it reaches a stage called blastocyst where a small cavity is formed in the embryo dividing it into an outer cell mass that forms the placenta in the future and an inner cell mass that forms the fetus. The number of cells the embryo contains at this stage exceeds a hundred. The blastocyst starts expanding until it hatches out of its shell, now ready to implant itself in the lining of the uterus. Implantation takes place 8-12 days after ovulation. The whole process of implantation could take up to four days for the embryo to completely imbed itself in the lining.
The interaction between the lining of the womb and the blastocyst can only occur during a limited time span known as “Window of implantation”. Out of this window the womb is considered to be non-receptive.
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Optimum receptivity is not only determined by hormonal factors or thickness of the lining of the womb but also relies on other factors such as immune mediation, growth factors and enzymes and other proteins present in the womb.
The first stage of implantation which is known as apposition and adhesion is when the blastocyst gets in contact with the implantation site of the lining of the uterus. The implantation site is usually in the upper two-thirds of the cavity of the uterus. However, it can attach itself anywhere else within the uterus cavity. The outer cell mass of the embryo starts attaching itself to the surface of the uterus lining then it starts digging its way into the deeper layers. This stage is called the invasion stage. The embryo also invades the blood vessels within the lining of the uterus. The aim of this process is to reconstruct these blood vessels in a manner to maintain an adequate blood flow between the uterus and the fetus, this process eventually forms the placenta later on.
Before implantation, the embryo takes its nutrients directly from the surrounding which is mainly secreted by uterus lining. At the early stages of implantation, it receives its nutrient directly from the surrounding cells of lining of the uterus. Eventually, the placenta would be its life support machine until delivery.
The early stages of implantation triggers rising levels of pregnancy hormones oestrogen, progesterone, and human chorionic gonadotropin causing the embryo to develop the placenta to form and the uterine lining to stay in place and support the pregnancy.
Most women experience no signs at implantation, however, some women may feel implantation symptoms including bleeding or spotting which is usually in the form brownish discharge occurring 1-2 days after a successful implantation. Some women may also feel cramping on the day of implantation, and cramping may continue after implantation. This is not particularly a warning sign, as in fact, cramping would occur on most women whether a woman feels it or not. The gentle contraction within the uterus may carry a benefit as it increases the blood supply to the womb and its lining. Some women experience changes in appetite and food preference. Breast discomfort and tenderness can also be associated with implantation which can last for few days. Other associated symptoms could be bloating, nausea, headaches, and mood swings, which are all related to hormonal changes.
It’s worth mentioning that when a woman is actively trying for pregnancy, usually the level of self-awareness is higher. This in itself can cause over interpretation of her symptoms as many of these symptoms are general and could be related to premenstrual tension rather than implantation.
Article was written by Mr Raef Faris, Gynaecology and Fertility Consultant at The Lister Fertility Clinic (part of HCA UK).
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