During this time, cells begin to differentiate into the various body systems. During the second trimester, most women begin to wear maternity clothes. Early obstetric ultrasound , comparing the size of an embryo or fetus to that of a reference group of pregnancies of known gestational age such as calculated from last menstrual periods , and using the mean gestational age of other embryos or fetuses of the same size. Gestational age of 40 weeks. During the time immediately after birth, both the mother and the baby are hormonally cued to bond, the mother through the release of oxytocin , a hormone also released during breastfeeding.
The uterus, the muscular organ that holds the developing fetus, can expand up to 20 times its normal size during pregnancy. Determination of which time point is to be used as origin for gestational age , as described in section above. Cell division continues at a rapid rate and the cells then develop into what is known as a blastocyst. During the time immediately after birth, both the mother and the baby are hormonally cued to bond, the mother through the release of oxytocin , a hormone also released during breastfeeding. Final weight gain takes place, which is the most weight gain throughout the pregnancy. Cell division begins approximately 24 to 36 hours after the male and female cells unite. After about ten weeks of gestational age, the embryo becomes known as a fetus. The development of the mass of cells that will become the infant is called embryogenesis during the first approximately ten weeks of gestation. Estimated date of confinement Due date estimation basically follows two steps: Start of gestational age According to American Congress of Obstetricians and Gynecologists , the main methods to calculate gestational age are: After the point of fertilization, the fused product of the female and male gamete is referred to as a zygote or fertilized egg. Fetal movement can become strong and be disruptive to the woman. Although the breasts have been developing internally since the beginning of the pregnancy, most of the visible changes appear after this point. A third point in time is also considered by some people to be the true beginning of a pregnancy: Studies show that skin-to-skin contact between a mother and her newborn immediately after birth is beneficial for both the mother and baby. Fertilization is the event where the egg cell fuses with the male gamete, spermatozoon. The placenta connects the developing embryo to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the last weeks of pregnancy. This typically happens in the fourth month, more specifically in the 20th to 21st week, or by the 19th week if the woman has been pregnant before. During pregnancy, the woman undergoes many physiological changes, which are entirely normal, including behavioral , cardiovascular , hematologic , metabolic , renal , and respiratory changes. Levels of progesterone and oestrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and therefore also the menstrual cycle. Adding the estimated gestational age at childbirth to the above time point. Relative size in 1st month simplified illustration Relative size in 3rd month simplified illustration Relative size in 5th month simplified illustration Relative size in 9th month simplified illustration Maternal changes Main article: Sex organs begin to appear during the third month of gestation. Maternal physiological changes in pregnancy Breast changes as seen during pregnancy. Pregnancy rates for sexual intercourse are highest during the menstrual cycle time from some 5 days before until 1 to 2 days after ovulation. The areolae are larger and darker.
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