The most common method to learn the date of birth (midwifery) has been passed from generation to generation since time immemorial. The fact that in ancient times the servants of Asclepius had no idea about ovulation, so the calculation was carried out, starting from the first day of the last menstrual period. Pregnancy, as such, this period is clearly no, but it has been estimated that the delivery is remote from this day exactly 280 days (40 weeks). This method of calculating the due date was at first accepted, and then firmly entrenched in obstetric practice.
The second popular method of determining the amount of time that will birth: the date of the first day of the last menstrual period, add seven days, and from the sequence number of month subtract three months. For example, the last menstruation began on 12 June (12.06.10). Then consider: 12+7=19. If Jun (6) to take three months, you get March (3). So, the expected due date – March 19 (19.03.11).
To more accurately determine the delivery date can be based on the date of ovulation - best time for conception. The lifespan of the egg one day, so the day after ovulation, conception will not occur. When the menstrual cycle lasts between 32 and 35 days, ovulation falls on day 16-18 at the start of menstruation. If the menstrual cycle lasts for 28 days, ovulation usually occurs on day 14. If the cycle is 21-24 days, ovulation falls on the 10-12 day.

Let's say your period started on June 10, the cycle is 28 days. Ovulation is expected 24 June, the right time for conception, from 19 to 24 June, 25 June pregnancy possible, and on 26 June - is unlikely.
If the menstrual cycle is 24 days, then ovulation occurs on June 20-21, conception is possible from 15 to 21-22 Jun.
When the menstrual cycle of 32 - 35 days ovulation predicted on 26-28 June, conception is possible from 21 to 28 June.

Ovulation can be determined much more precisely, if systematically measure the basal temperature (in the rectum). It is recommended to measure in the morning at the same time, getting out of bed. A thermometer is introduced for 10 minutes into the rectum to 5 cm With a daily measurement graph of basal temperature not exceeding 37 degrees before ovulation, and rises after. Ovulation occurs the day before the temperature rise.
To determine the date of childbirth and in the first movement of the fetus. To the date of the first stirring is added 20 weeks (for nulliparous women) and 18 weeks (for potamonidae). But this method of calculating the date of birth is even more doubtful than the previous one. Some women begin to feel fetal movement much earlier than the set time (20 weeks) and others a little later. Actually it depends on the activity of the baby, locate it in the uterus, the level of sensitivity of the uterine walls, and some doctors say that very often, a woman confuses an ordinary gas with the first stirring of the fetus.