What happens in the first week of pregnancy?

A woman’s pregnancy is clinically dated from the first day of her last period, however the exact moment of conception, although difficult to pinpoint accurately, is likely to be around a fortnight later. So when a woman is considered three weeks pregnant, the embryo is only actually one week old in terms of growth following conception, and will have only just implanted, or maybe even not have implanted yet. At this stage a positive pregnancy test is very unlikely.

The fertilized egg is known as a zygote, and conception usually occurs within the fallopian tubes. The fertilized egg divided into two cells after about 12 hours, and after about 30 hours divides again into four cells. It slowly travels down the Fallopian tube, moved gently by tiny muscular contractions and tiny hair like projections called cilia.

Over the next few days the zygote continues to divide in a biological process called mitosis until it becomes a hollow ball of cells called a blastocyst. Around this time it reaches the uterus and starts to implant itself into the endometrium (the womb lining). This takes place some time from day 6 to day 12.

Many pregnancies will already have failed by this point; the University of California states that half of all fertilized eggs fail before the time of the woman’s next period by either not implanting or not continuing to grow. https://www.ucsfhealth.org/education/conception-how-it-works#:~:text=Adhesive%20sites%20on%20the%20cilia,tube%20takes%20about%2030%20hours. Even if the blastocyst does get this far, healthcare professionals believe that up to 25% of all pregnancies ends in miscarriage, usually within the first few weeks.

The hormone progesterone, which rises significantly during the second half of a woman’s menstrual cycle, is very important in preparing the endometrium to receive a blastocyst. It encourages the womb lining to thicken and become rich with blood ready for a pregnancy, and stimulates the release of important nutrients.

What is implantation bleeding?

A minority of women experience some light bleeding or spotting for a couple of days when implantation occurs, however most women are unaware that this process is happening and have no symptoms.

A pregnancy test would not yet be positive; this can take days or even a further week or two. Pregnancy tests work by detecting the hormone HCG (human chorionic gonadotrophin) in the urine. This hormone is produced by the trophoblast cells which are the outer layer of cells in the blastocyst which eventually will become the placenta. The HCG hormone tells the ovaries to stop releasing further eggs, and stimulates the corpus luteum (a normal cyst which develops in the ovary which has released the successful egg) to continue to produce progesterone for the next few weeks.

Around this time an inner cell mass is developing inside the trophoblast. Also known as the embryoblast, this is the beginnings of a tiny human and will start to form further in week 2. At this stage the structure only measures around 0.1 to 0.2 millimetres across. The trophoblast outer layer becomes tightly packed in a process called compaction. This forms a barrier between the embryo and the uterus, protecting it from toxins and making it less vulnerable to the mother’s immune system, as the placenta will continue to do further along. The placenta develops from the trophoblastic cells, and by the end of the first trimester this will take over hormone production, pass nourishment to the foetus and filter out toxins and pathogens.

Following implantation, further hormones are secreted including HCS (human chorionic somatommotropin) and HPL (human placental lactogen) which both help nourish the new embryo.

The inner cell mass continues to develop, differentiating into three different cell layers. The ectoderm develops into the nervous system, skin and hair. The endoderm develops into internal organs such as the digestive system and the respiratory system. The mesoderm becomes the bones, muscles and blood vessels.

How can scientist see a developing blastocyst?

There are several imaging techniques used by researchers to study early embryo development. Time-lapse microscopy involves taking a series of images over a period of time to observe changes such as cell division. Important learning regarding the mechanics of early cell differentiation has been possible due to time-lapse microscopy in vitro (cells in a dish under laboratory conditions).Immunostaining and confocal microscopy is a technique which involves specific proteins being marked by fluorescent dyes in order to visualise different cell types.

Ultrasound is not helpful at this stage of a pregnancy; a blastocyst is far too small to be visualised on a scan; usually the earliest an embryo can be seen is at around 5-6 weeks pregnant (3-4 weeks embryo growth from conception). However, ultrasound can be used to assess the endometrium at this stage, measuring the thickness and vascularity in terms of whether it is at an appropriate stage to receive a pregnancy successfully. This can be particularly helpful in IVF treatments before an embryo transfer is undertaken.

How big is an embryo at 2 weeks pregnant?

Once the blastocyst has implanted, considerable development occurs during days 7-14 following conception. Chorionic villi, finger like projections, grow from the trophoblast into the uterine wall ensuring maximum contact with the mother’s blood. Implantation is complete by day 9 or 10.

Around now a primitive circulatory system begins to develop. A yolk sac is formed which will feed the embryo and supply blood for the first few weeks until the placenta is formed. Neither the yolk sac nor the embryo are usually visible on ultrasound until the woman is around 5 weeks pregnant (3 weeks post fertilisation). The amnion, a membrane which will surround the embryo and it’s surrounding fluid also begins to form. This is a critical stage in the development of the new life and around now the blastocyst is particularly vulnerable to any toxins or pathogens that may be in it’s environment. Two weeks in, there is still no recognizable embryo as such, but there soon will be.

When are embryos transferred in IVF treatment?

Embryo transfer most commonly takes place on day 5 of development, on day 18 or 19 following the start of the woman’s last period. By then, the blastocyst has formed and can be assessed by clinicians as to the quality of the embryo which has developed. However, transfer can take place from day 2-day 6 depending on clinical scenarios.