Women should be able to feel the uterus at about 3 inches (6.6 cm) below the belly button; this is the beginning of the “baby bump” (abdominal swelling due to an expanding uterus) in some women. Many sperm are released with the possibility of just one sperm cell managing to adhere to and enter the thick protective shell-like layer surrounding the ovum. It remains in the fallopian tube for about three days. The sperm and egg unite in one of your fallopian tubes to form a one-celled entity called a zygote.
At twenty weeks, the developing baby is about 6 inches long (13.2 cm) and may weigh about 10 ounces. The first sperm that penetrates fully into the egg donates its genetic material (DNA). The egg then polarizes, repelling any additional sperm. The fertilized egg (called a blastocyte) continues to divide as it passes slowly through the fallopian tube to the uterus where its next job is to attach to the endometrium (a process called implantation). Before this happens, the blastocyte breaks out of its protective covering. If more than one egg is released and fertilized, you might have multiple zygotes. The baby may begin to make movements that the mother can feel at about 19 to 21 weeks; this baby movement is termed “quickening”.
The beginning mark
The resulting combination is called a zygote, a new and genetically unique organism. When the blastocyte establishes contact with the endometrium, an exchange of hormones helps the blastocyte attach. The zygote typically has 46 chromosomes — 23 from you and 23 from the father. The baby at this stage of development can move its facial muscles, yawn, and suck its thumb. The term “conception” refers variably to either fertilization or to formation of the conceptus after its implantation in the uterus, and this terminology is controversial.
Some women notice spotting (or slight bleeding) for one or two days around the time of implantation. These chromosomes help determine your baby’s sex, traits such as eye and hair color, and, to some extent, personality and intelligence. The expanding uterus at 20 weeks is felt at the level of the belly button. Prior to fertilization, each ovum, as a gamete, contains half of the genetic material that will fuse with the male gamete, which carries the other half of the genetic material (DNA). The ovum only carries the X female sex chromosome whilst the sperm carries a single sex chromosome of either an X or a male Y chromosome. The endometrium becomes thicker and the cervix is sealed by a plug of mucus.
Soon after fertilization, the zygote travels down the fallopian tube toward the uterus. In the US, women that have prenatal care usually have an ultrasound done at 20 weeks to determine that the placenta is attached normally and that the baby is developing without any problems. The resulting human zygote is similar to the majority of somatic cells because it contains two copies of the genome in a diploid set of chromosomes. Within three weeks, the blastocyte cells ultimately form a little ball, or an embryo, and the baby’s first nerve cells have already formed. At the same time, it will begin dividing to form a cluster of cells resembling a tiny raspberry — a morula.
The baby’s movements can be seen with Doppler imaging, and usually the sex of the baby can be determined at this time, so if you want to be surprised about the sex of your baby at delivery, let your doctor know before the Doppler ultrasound is started! One set of chromosomes came from the nucleus of the ovum and the second set from the nucleus of the sperm. Your developing baby is called an embryo from the moment of conception to the eighth week of pregnancy. By the time it reaches the uterus, the rapidly dividing ball of cells — now known as a blastocyst — has separated into two sections. At 24 weeks, the baby may weigh 1.4 pounds and can respond to sounds.
The zygote is male if the egg is fertilized by a sperm that carries a Y chromosome, and it is female if the egg is fertilized by a sperm that carries an X chromosome. After the eighth week and until the moment of birth, your developing baby is called a fetus. The inner group of cells will become the embryo. Doppler studies show the sound response by measuring movement and heartbeat rates. The Y chromosome contains a gene, SRY, which will switch on androgen production at a later stage, leading to the development of a male body type.
The development stages of pregnancy are called trimesters, or three-month periods, because of the distinct changes that occur in each stage. The outer group will become the cells that nourish and protect it. Sometimes the baby will develop hiccups that the mother can feel! In contrast, the mitochondrial genetic information of the zygote comes entirely from the mother via the ovum. As the fertilized egg grows, a water-tight sac forms around it, gradually filling with fluid.
On contact, the blastocyst will burrow into the uterine wall for nourishment. The baby’s inner ear canals are developed at 24 weeks, so researchers speculate the baby can sense its position in the uterus. The embryonic period in humans begins at fertilization (penetration of the egg by the sperm) and continues until the end of the 10th week of gestation (8th week by embryonic age). The period of two weeks from fertilization is also referred to as the germinal stage. This is called the amniotic sac, and it helps cushion the growing embryo. This process is called implantation.
Moment before delivery
At 28 weeks, the baby normally weighs about 2 and one-half pounds and has developed to the point that if the baby is birthed prematurely for any reason, the chances are good that the infant will survive, but usually would require a hospital stay. The embryo spends the next few days traveling down the Fallopian tube. The placenta also develops. The placenta, which will nourish your baby throughout the pregnancy, also begins to form. Your doctor may discuss signs of premature labor and suggest you (and your partner) take classes on what to do at the time of delivery of your full-term baby. It starts out as a single cell zygote and then divides several times to form a ball of cells called a morula.