A full term pregnancy lasts 40 weeks (280 days), and is divided into
three time periods of about 13 1/3 weeks or 3 calendar months known as
trimesters. Each trimester serves as an important milestone in the
development of the fetus and progress of the pregnancy.
During the first trimester the sex of the baby is determined, the baby's
organs are created, and twins may be formed.
You may experience fatigue, nausea, headaches, and swollen or tender
breasts. Your blood pressure will decrease and your heart rate will
begin to increase as your body begins to accommodate the needs of your
developing baby .
You may expect to gain 4 to 5 pounds during the first trimester
unless you have significant nausea and vomiting. An additional 300 Calories
per day are recommended
during a normal singleton pregnancy.
Gestational Week 1 and 2 :
During the first two weeks after your last
menstrual period egg follicles in the ovaries mature under the
stimulus of follicle-stimulating hormone (FSH) a hormone secreted by the pituitary gland
in the brain. High levels of another hormone estradiol, produced by the developing egg follicle,
of luteinizing hormone (LH) ,yet another hormone from the pituitary gland. LH causes release of the egg
from its follicle (ovulation) around day 14 of the menstrual cycle. Some women
may experience slight pain with ovulation (mittle schmertz).
The empty egg follicle develops into a yellow
colored structure called the corpus luteum (CL). The corpus luteum secretes
progesterone which prepares the lining of the uterus for implantation by the
embryo and causes a rise in basal body temperature. If the released egg is
fertilized, then the fertilized egg will begin producing
hormone human chorionic gonadotropin
maintain the corpus luteum until the placenta is able to produce enough
progesterone to support the pregnancy.
Your due date may be
estimated by adding seven days to the first day of your last menstrual
period (LMP) and then subtracting three months.
This method of estimating your due
date depends on accurate recall of your last menstrual period, assumes
you have regular 28 day cycles, and that ovulation and fertilization
occurred on day 14 of your cycle. If you do not have regular menstrual
cycles, or you do not ovulate when expected a more accurate estimation
of your due date may be established by using ultrasound to measure the size of your baby before 20 weeks.
The duration of pregnancy, called the
gestational age or menstrual age, is expressed as completed weeks since
the first day of your last menstrual period. For example a calculated age of
25 weeks and 2 days or 25 and 6 days from the first day of the last
menstrual period would both be considered to be 25 completed weeks .
Conceptual age is a term used by persons who study the development of
the embryo and fetus. The conceptual age is the time elapsed since ovulation and
fertilization of the egg. Since the first day of ovulation occurs about 2 weeks after the
first day of the LMP, the conceptual age is 2 weeks less than the
1. Allen J Wilcox, David Dunson, and Donna Day Baird. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study
BMJ 2000; 321: 1259-1262.MEDLINE