Prenatal Development

How your due date is determined

Your physician calculates your due date (and your baby's gestational age) starting from the first day of your last menstrual period (LMP). Pregnancy actually lasts 38 weeks from the time conception takes place, but since it is usually difficult to determine exactly when the egg & sperm merged, physicians approximate your due date to be 40 weeks after the first day of your last menstrual period.

First Trimester

Week 1: Interestingly enough, you're actually not pregnant the first week or two of the total time allotted toward your pregnancy since (1) doctors include the time since the first day of your last menstrual period, and (2) conception typically takes place about 2 weeks later.

Week 2: Conception takes place when the sperm penetrates the egg's outer membrane. This fertilized egg is temporarily called a zygote. Approximately 3-4 days later it will reach the uterus and be called an embryo.

Week 4: Once in the uterus, the embryo may freely float around for up to 48 hours. Upon implantation, a placenta forms between embryo and the mother. This placenta will transport nourishment to the embryo and waste products away from it for the entire length of the pregnancy. From now until week 10, the embryo will be most vulnerable to anything which might interfere with its development (i.e. alcohol, radiation, or infectious diseases).

Week 5: The embryo is about 1/17 of an inch long. Its heart & circulatory system are taking shape. The beating heart may be detected with an ultrasound exam.

Week 6: By this time, the embryo is about 1/8 of an inch long with the heart beating a regular rhythm. The digestive & respiratory systems are beginning to form and the head & mouth are beginning to appear.

Week 8: The embryo is about 1/2 inch long. Wrists, ankles & elbows are beginning to be visible. Its heart is pumping around 150 beats per minute.

Week 10: The embryo is now 1 inch long and is called a fetus. Hands, fingers, feet & toes are visible. Facial features are also developing. Many of the internal organs are now functioning and the nervous system is beginning to function also.

• Prenatal Development - First Trimester
• Prenatal Development - Second Trimester
• Prenatal Development - Third Trimester