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| Home > Labour and Birth > Basic Concepts for Understanding Labor
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| | Basic Concepts for Understanding Labor
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Descent
Descent is the beginning of the baby`s movement down through the pelvis. This happens during late pregnancy or labor. The baby is said to be floating, when the lowest part is still above the level of the pubic bone. Some descent-either gradual or sudden may take place several weeks before the onset of labor for primigravidas (women pregnant for the first time). For multigravidas (women who have been pregnant more than once), labor begins when the baby is still floating.
The progress of the baby`s descent is measured in terms of station. Station refers to the placement of the presenting part (the body part that comes first-usually the baby`s head) in relation to the ischial spines the bones marking the middle of the true pelvis.The middle pelvis is considered as 0 station. For example, if the top of the head is at 0 station, it means that it has descended to the middle of the pelvis. A floating head at minus ( - ) 4 station implies four centimeters above the mid pelvis. The minus signs indicate the position above the middle pelvis, while the plus sign indicates the position below middle pelvis. The positions vary from -4 to +4. When the head is at the vagina and on its way out, it is at a + 4 station. Many women begin labor at a - I, 0, or + 1 station, meaning that some descent has already taken place.
Descent is also described by "lightening," meaning the relief of pressure in the women`s chest and stomach when her baby moves down. "Dropping" implies physically noticeable descent. "Engagement" means that the presenting part is "engaged," or at about 0 station, and fixed in the pelvis.
Presentation and Position
The terms presentation, position refers to the position and direction of the baby, lying inside your uterus. The term Presentation denotes the part of the baby that is lying over the cervix. The vertex presentation is the most favorable and most common presentation (occurring 95 percent of the time) is, when the top of the baby`s head rests down over the cervix. Other presenta-tions are the frank breech (buttocks), footling breech (feet), complete breech (buttocks and feet), shoulder, face, and brow presentations. Position refers to the direction in which the baby lies within your body. The positions are anterior, referring to your front; posterior, your back; and transverse, your side.
If your doctor or midwife tells you the baby is occiput anterior (OA), it means that the back of the baby`s head (the occiput) is pointing toward your anterior (front). Here are some other com-mon descriptions of the baby`s presentation and position:
Left Occiput Anterior (LOA): The back of the baby`s head toward your left anterior Right Occiput Anterior (ROA): The back of the baby`s head toward your right anterior .
Occiput Posterior (OP): The back of the baby`s head directly toward your posterior .
Left Occiput Posterior (LOP): The back of the baby`s head toward your left posterior
Right Occiput Transverse (ROT): The back of the baby`s head toward your right side.
Left Occiput Transverse (LOT): The back of the baby`s head toward your left side.
Right Sacrum Anterior (RSA): The baby`s tailbone or buttocks (sacrum) toward your right front. This is how a breech presen-tation is described.
Ripening, Effacement and Dilatation
These terms refer to changes in the cervix that begin gradually before labor starts and end just when the baby is about to be born.
Ripening: The cervix is usually firm in females. The process of softening of the firm cervix, beginning in late pregnancy is known as ripening.
Effacement: It is the thinning of the cervix. For a primigravida, a substantial degree of effacement has usually taken place before dilatation (opening) begins. Multigravidas usually have effacement in conjunction with dilatation. Effacement is measured during vaginal examination and calculated in percentages. The effacement can vary from zero to hundred percent. "Zero percent effacement" means the cervix has not begun to thin; 50 percent effacement" means the cervix has thinned about halfway; "100 percent effacement" means the cervix has thinned completely.
Dilatation: By dilation we mean the opening of the cervix. The cervix may dilate slightly before the onset of labor but most dilatation takes place during labor. A vaginal exam estimates dilation which is measured in centimeters. At most, dilation can reach up to 10 centimeters. When the cervix is opened only a fingertip, it is one centimeter dilated: at the halfway point it is five centimeters; and when fully dilated it is ten centimeters dilated.
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