During pregnancy, maternal blood is the central support system for a developing fetus, supplying everything it needs to grow while also removing waste. The health and composition of this blood are so connected to the pregnancy’s progression that it is monitored through various screenings. These evaluations help ensure the well-being of both mother and baby from early development through delivery.
The Role of Maternal Blood in Fetal Development
Maternal blood is the source of life-sustaining substances for the fetus, though their circulatory systems remain separate. This exchange occurs within the placenta, a temporary organ that attaches to the uterine wall. The placenta acts as an interface where maternal and fetal blood vessels come into close proximity without mixing, allowing for efficient transfer of materials. Through this connection, the mother’s blood delivers oxygen, glucose, amino acids, vitamins, and minerals into the fetal circulation. Simultaneously, the placenta facilitates the removal of waste products like carbon dioxide and urea from the fetal bloodstream into the mother’s circulation, where her body can eliminate them.
The placenta also serves as a protective barrier and an endocrine organ. It produces hormones like progesterone and human placental lactogen (hPL) to maintain the pregnancy and prepare the mother’s body for breastfeeding. As delivery approaches, the placenta transfers antibodies from the mother to the fetus, providing passive immunity that protects the newborn.
Physiological Changes in Maternal Blood
To accommodate the demands of pregnancy, the maternal circulatory system undergoes significant adaptations. A primary change is a 30-50% increase in blood volume, which begins as early as the sixth week. This expansion is necessary to support the enlarged uterus, the placenta, and the growing fetus, ensuring adequate blood flow and oxygenation.
This volume increase is driven by a 40-50% expansion of blood plasma, the fluid component of blood. The number of red blood cells also increases, but at a slower rate of 15-25%. This disparity leads to physiological anemia, or hemodilution, where the concentration of red blood cells is lower than in a non-pregnant state.
In preparation for childbirth, the mother’s blood also becomes more prone to clotting. Levels of several clotting factors, including fibrinogen and factors VII, VIII, IX, and X, are elevated. This hypercoagulable state is a protective mechanism intended to minimize blood loss during delivery, but it also slightly increases the risk of blood clots.
Standard Maternal Blood Screening
Throughout pregnancy, routine blood screenings monitor the health of both mother and fetus. Early in the first trimester, initial tests establish a baseline and check for pre-existing conditions. Common screenings performed at various stages include:
- A complete blood count (CBC) to assess red and white blood cell levels for signs of anemia or infection.
- Blood typing and Rh factor determination to identify any potential incompatibility between mother and baby.
- Screening for infectious diseases like syphilis, hepatitis B, and HIV, and verifying immunity to rubella and varicella.
- A first-trimester screen (11-14 weeks) that combines a blood test and an ultrasound to assess risk for chromosomal conditions like Down syndrome.
- A second-trimester maternal serum screen (15-20 weeks), or quad screen, which measures four substances in the blood to assess risk for Down syndrome, trisomy 18, and neural tube defects.
- A glucose screening test (24-28 weeks) to check for gestational diabetes. This test involves drinking a sugary solution followed by a blood draw to measure how the body processes sugar.
Non-Invasive Prenatal Testing
Non-Invasive Prenatal Testing (NIPT) is an advanced screening method that analyzes fetal DNA circulating in the maternal bloodstream. During pregnancy, small fragments of placental DNA, known as cell-free DNA (cfDNA), enter the mother’s blood. This cfDNA reflects the fetal genetic makeup, allowing for early screening without invasive procedures.
NIPT can be performed as early as 10 weeks and is highly accurate for detecting common chromosomal conditions like Trisomy 21 (Down syndrome), Trisomy 18, and Trisomy 13. The test can also screen for abnormalities in the sex chromosomes and determine the sex of the fetus.
The process involves a simple blood draw from the mother. Technicians analyze the cfDNA to detect quantitative differences in DNA from specific chromosomes; for example, a higher amount of DNA from chromosome 21 suggests an increased risk for Down syndrome. The proportion of fetal DNA in the mother’s blood is a factor in the test’s accuracy.
NIPT is a screening test, not a diagnostic one. A high-risk result indicates an increased chance of a condition but is not a definitive diagnosis. Confirmatory diagnostic tests, such as amniocentesis or chorionic villus sampling (CVS), are recommended to confirm an abnormal NIPT result.
The Importance of Blood Type and Rh Factor
Determining the mother’s blood type and Rh factor is a component of early prenatal screening. The Rh factor is a protein on the surface of red blood cells; people with it are Rh-positive, and those without are Rh-negative. This difference is significant if an Rh-negative mother carries an Rh-positive fetus, which can lead to Rh incompatibility.
If a small amount of the fetus’s Rh-positive blood enters the mother’s bloodstream, her immune system may produce antibodies against the Rh protein. This process, called sensitization, usually does not affect the first pregnancy, as the baby is often born before a significant antibody response develops. However, these antibodies can pose a risk to subsequent pregnancies with an Rh-positive fetus, as the mother’s immune system can attack the fetal red blood cells.
To prevent this, a medication called Rho(D) immune globulin (RhoGAM) is administered. This injection prevents the mother’s immune system from producing its own Rh antibodies. For Rh-negative mothers, the shot is given around the 28th week of pregnancy and again within 72 hours after delivery if the baby is confirmed to be Rh-positive.