Pregnancy happens when sperm fertilizes an egg, the fertilized egg travels to the uterus, successfully implants in the uterine lining, and then develops over about 40 weeks into a baby.

What pregnancy is

Pregnancy is the period when an embryo and then a fetus grows inside the uterus after conception (fertilization plus implantation). It typically lasts around 40 weeks from the first day of the last menstrual period and is divided into three trimesters.

Step 1: Ovulation and egg release

  • Each month, hormones stimulate the ovaries to mature several follicles, usually leading to the release of one egg (ovulation).
  • Around mid‑cycle, a surge of luteinizing hormone (LH) triggers the follicle to burst and release the egg into the abdominal cavity, where the fimbriae of the fallopian tube sweep it into the tube.

Step 2: Sperm entering the body

  • During penis‑in‑vagina sex without effective contraception, ejaculation releases semen containing millions of sperm into the vagina.
  • The sperm swim through the cervix, into the uterus, and up the fallopian tubes; they can survive in the reproductive tract for up to about 5–6 days, waiting for an egg.

Step 3: Fertilization (conception begins)

  • Fertilization usually occurs in the ampulla of the fallopian tube when one sperm penetrates the outer layers of the egg; once that happens, the egg’s surface changes so no other sperm can enter.
  • The fertilized egg is now called a zygote and quickly starts dividing into more cells as it travels down the tube toward the uterus over several days.

Step 4: Implantation (pregnancy officially starts)

  • After about 5–7 days, the ball of cells (now a blastocyst) reaches the uterus and, if conditions are right, attaches to and burrows into the thickened uterine lining; this is implantation and is the medical starting point of pregnancy.
  • Once implanted, it begins forming the placenta and releases human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests, which can appear in blood roughly 11 days after conception and later in urine.

Step 5: Hormones and body changes

  • Progesterone and other hormones rise to maintain the uterine lining, support early development, and prevent menstruation, which is why periods usually stop in pregnancy.
  • As pregnancy continues, the placenta takes over many hormone‑producing roles, supporting fetal growth and changing the pregnant person’s metabolism, blood volume, and organs’ workload.

Development over three trimesters

  • First trimester: Organs begin to form; the embryo becomes a fetus; miscarriage risk is relatively higher in this period.
  • Second trimester: Fetus grows larger and more active; many people begin to feel movement, and detailed anatomy scans are performed.
  • Third trimester: Rapid growth, lung maturation, and preparation for birth; the fetus usually turns head‑down before labor.

Why it does not always lead to pregnancy

  • Not every act of sex leads to pregnancy because ovulation only occurs on certain days, sperm may not reach the egg, or fertilization may fail.
  • Even after fertilization, a significant number of embryos never successfully implant and are lost before or around the time of an expected period, often without being noticed.

TL;DR: Pregnancy works through a coordinated sequence: ovulation, sperm reaching the egg, fertilization in the tube, travel of the fertilized egg to the uterus, and successful implantation with rising hormones that support about 40 weeks of fetal development.

Information gathered from public forums or data available on the internet and portrayed here.