The first thing most women notice is that their period doesn't arrive. Find out about the signs and symptoms of pregnancy. By the time you're 8 weeks pregnant, you'll probably have missed your second period. However, some women experience a little bleeding during the early weeks of pregnancy.
http://www.nhs.uk/conditions/pregnancy-and-baby/pages/pregnancy-weeks-4-5-6-7-8.aspx?tabname=labour%20and%20birth
The placenta (also known as afterbirth) is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, provide thermo-regulation to the fetus, waste elimination, and gas exchange via the mother's blood supply, fight against internal infection and produce hormones to support pregnancy.
https://en.wikipedia.org/wiki/Placenta
Week 8 of pregnancy. By the time you're 8 weeks pregnant, the baby is called a foetus, which means offspring. The legs are getting longer. The different parts of the leg aren't properly distinct yet – it will be a bit longer before the knees, ankles, thighs and toes develop.
http://www.nhs.uk/conditions/pregnancy-and-baby/pages/pregnancy-weeks-4-5-6-7-8.aspx?tabname=labour%20and%20birth
The placenta attaches to the wall of the uterus, and the baby's umbilical cord develops from the placenta. The umbilical cord is what connects the mother and the baby. Placentas are a defining characteristic of placental mammals, but are also found in some non-mammals with varying levels of development.
https://en.wikipedia.org/wiki/Placenta
What does the placenta do? The placenta is an organ that develops in your uterus during pregnancy. This structure provides oxygen and nutrients to your growing baby and removes waste products from your baby's blood. The placenta attaches to the wall of your uterus, and your baby's umbilical cord arises from it. In most pregnancies, the placenta attaches at the top or side of the uterus. Various factors can affect the health of the placenta during pregnancy, some modifiable and some not.
http://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/placenta/art-20044425
The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to a capacitor. Ultrasound image of human placenta and umbilical cord (color Doppler rendering) with central cord insertion and three umbilical vessels, at 20 weeks of pregnancy.
https://en.wikipedia.org/wiki/Placenta
Week 4 of pregnancy. In weeks 4-5 of early pregnancy, the embryo grows and develops within the lining of the womb. The outer cells reach out to form links with the mother's blood supply. The inner cells form into two, and then later into three layers.
http://www.nhs.uk/conditions/pregnancy-and-baby/pages/pregnancy-weeks-4-5-6-7-8.aspx?tabname=labour%20and%20birth
Even before a woman is noticeably pregnant, her intake of food and beverage passes through the developing placenta and umbilical cord to nourish (or harm) the growing embryo. The use of harmful substances during pregnancy is doubly damaging because both the woman and embryo / fetus may suffer significant harm. Following implantation, maternal and embryonic tissues combine and begin to form the placenta (pla-sen’ta).6. Figure 2.3 - The Umbilical Cord & the 5½- Week Embryo.
http://www.ehd.org/dev_article_unit2.php
Unit 2: 1 to 2 Weeks. Implantation | Amnion and Yolk Sac | HCG | Pregnancy Tests | Placenta | Umbilical Cord. Closer Look: Applying the Science: Implantation — Penetration of the Uterine Wall. The primary event of the second week is implantation. The attachment phase of implantation is briefly discussed in Unit 1.
http://www.ehd.org/dev_article_unit2.php
The best type of physician to consult with about this is an OBGYN. The placenta is the structure that links the maternal uterus with the growing fetus. The placenta begins forming at the point of implantation, which is about 8 days after the point of conception. Nutrient exchange between mother and fetus begins at this time, though the circulation of the placenta does not fully develop till the end of the first trimester. Unfortunately, there are no clear guidelines to say when during the pregnancy consuming alcohol is or is not harmful to the fetus.
https://www.zocdoc.com/answers/4307/how-soon-into-pregnancy-does-placenta-form
when does the umbilical cord form during pregnancy
The umbilical cord forms at 20 weeks of pregnancy.