Two points of difference between asexual and sexual reproduction are as follows:
(a) A single parent is involved in asexual reproduction, while two parents are involved in sexual reproduction.
(b) Meiosis does not happen in asexual reproduction during gamete formation.
reason of variations in offspring produced by sexual reproduction: Two sets of genes are contributed by different parents, which bring different sets of characters in the offspring.

Gamete is formed as a result of meiosis in gametogenic cells. Zygote is formed after fusion of two different kinds of gametes. Different types of gametes are formed by different parents. Zygote is formed in the female reproductive part only.
Role of gamete in sexual reproduction: Gamete formation results in halving of number of chromosomes which is necessary to maintain the number of chromosomes in an organism which reproduces sexually.
Role of zygote in sexual reproduction: Zygote formation makes the number of chromosome equal to the somatic cells. further, it is the zygote which undergoes mitosis to develop into an embryo and subsequently into a new individual.

There are various methods to avoid pregnancy. The physical barrier methods prevent the entry of sperm into the uterus. The hormonal pills prevent transfer of egg to the uterus. Copper-T is a method which prevents entry of sperms in the fallopian tube and thus prevents fertilization. Surgical methods are permanent methods to stop the chances of pregnancy.
Copper-T : Copper-T is in the shape of 'T' and is made of copper. Copper is known to have contraceptive properties. Moreover, copper-T makes a barrier in the at the opening of the fallopian tubes in the uterus and thus prevents sperms from entering the fallopian tubes to prevent fertilization.

During the ovulation cycle, one egg is released by either of the ovaries. The egg reaches the fallopian tube. The sperm enters the vagina during mating. From there, it enters the uterus through cervix and reaches fallopian tube. Numerous sperms reach the fallopian tube but only one sperm is required for fertilization.
Since only one egg is released in a single ovulation cycle and ovulation cycle happens once in a month, so fertilization occurs once in a month.

(a) Special tissue connection between embryo and uterine wall.
(b) Possesses villi that increase the surface area.
(c) Facilitate passage of nutrition and oxygen to embryo from mother through blood.
(d) Waste substances produced by embryo are removed through placenta into mother's blood.