Changes and Symptoms in First Month/Trimester of Pregnancy

9 Week Human Embryo from Ectopic Pregnancy
As you’re changing and growing, so is the baby inside of you. It’s difficult to imagine just how much growth is packed into such a short period of time. In the first three months, the fetus is only 8–10 millimeters in length (the width of your thumb), but within that embryo is a beating heart with blood circulating through it and the potential for growth with the maturation of multiple organ systems. At this point, it’s difficult to determine the gender, but limb buds that will eventually change and become arms and legs are clearly visible. Doctors can also identify the cranial (head) portion of the fetus. They will use this crownrump length from an ultrasound (roughly the distance between the head portion of the embryo and its bottom side) as a form of measurement to determine how far along the pregnancy is. It’s actually fairly accurate when measured.
Physiological Changes in the Baby’s Growth
When the sperm meets the egg, a virtual explosion occurs in terms of rapid cellular division. By the end of the first trimester, most of the baby’s organ systems are formed, including the heart, lungs, kidneys, and digestive system. Still forming is the baby’s neurological system, including the brain, brain stem, and spinal cord. Organ formation occurs through most of the first trimester.
Afterward, the organ systems that have formed are increasing in size and maturation By about the fifth or sixth week of pregnancy, heart motion is visible with a trans-vaginal ultrasound, although most of the other systems are too small to see at this time. A fluid sac has developed around the fetus that will eventually comprise the amniotic fluid. The yolk sac is present in early pregnancy, although it slowly regresses and the placenta takes its place as the major support system of nutrients and the exchange of waste products.