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Gestational age: 5 weeks and 0 days until 5 weeks and 6 days old. 36–42 days from last menstruation. Embryonic age: Week nr 4. 3 weeks old. 22–28 days from fertilization. The embryo measures 4 mm (1 ⁄ 8 in) in length and begins to curve into a C shape. The heart bulges, further develops, and begins to beat in a regular rhythm.
In most successful pregnancies, the embryo implants 8 to 10 days after ovulation. [7] The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception". Rapid growth occurs and the embryo's main features begin to take form.
Developmental milestones [3] [4] Age Motor Speech Vision and hearing Social 1–1.5 months When held upright, holds head erect and steady. Cooes and babbles at parents and people they know Focuses on parents. Loves looking at new faces; Starts to smile at parents; Startled by sudden noises; Recognition of familiar individuals; 1.6–2 months
The normal period of gestation (pregnancy) is about nine months or 36 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days. [1] During this stage, the zygote divides in a process called cleavage.
In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days. [30] The average time to birth has been estimated to be 268 days (38 weeks and two days) from ovulation, with a standard deviation of 10 days or coefficient of variation of 3.7%. [32]
The average age of a girl's first period is 12 to 13 (12.5 years in the United States, [6] 12.72 in Canada, [7] 12.9 in the UK [8]) but, in postmenarchal girls, about 80% of the cycles are anovulatory in the first year after menarche, which declines to 50% in the third year, and to 10% by the sixth. [9]
There is also considerable variability in this interval, with a 95% prediction interval of the ovulation of 9 to 20 days after menstruation even for an average woman who has a mean LMP-to-ovulation time of 14.6. [9] In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days. [8]
Ovulation occurs ~35 hours after the beginning of the LH surge or ~10 hours following the LH surge. Several days after ovulation, the increasing amount of estrogen produced by the corpus luteum may cause one or two days of fertile cervical mucus, lower basal body temperatures, or both. This is known as a "secondary estrogen surge".