Baby development at week 1

It can be difficult to know exactly when conception takes place, so the beginning of pregnancy is counted from the first day of the last menstrual period - which is about two weeks before conception even takes place. After menstruation, the production of hormones in the pituitary gland increases. The hormone affects the ovaries and causes an egg to mature inside a bladder in the ovary. In the egg bladder, the hormone estrogen is formed. When the bladder ruptures, the egg loosens and the egg bladder is transformed into a so-called corpus luteum. The corpus luteum produces corpus luteum hormone, or progesterone, an important hormone that causes the lining of the uterus to grow and get ready to receive a fertilized egg.

In assisted fertilisation (IVF), the beginning of pregnancy is calculated two weeks before the embryo is inserted into the uterus. The egg retrieval is counted as week 2+0 of the pregnancy.