GarbhaDhana. All sources recognize this as the first Sanskar. It consists of rites performed before conception in the belief that it ensures a healthy child. This is the enthusiastic prayer for a child. This is done for fulfillment of parental duty to continue the race. This Sanskar is performed by husband. With conception, the seed and all the other impurities are destroyed. The atman stays 55 days inside the father and 270 days inside mother before birth. 

1. Thursday for male conception. 
2. Monday and  Friday for female conception. 
3. Uttara, Uttarashada, Uttarabhadrapad, Mrgasiras, Hastha, Anuradha, Rohini, Svati, Shravan, Dhanishta and Shatataraka - 1st choice. 
4. Chitra, Punarvasu, Pushya and Asvini - Medium 2nd choice.
   
5. Night Time [Sunset + 3 hours] to [Sunrise - 3 hours]
6. Vrishab, Mithun, Kark, Sinh, Kanya, Thula, Dhanu and Meen Lagna.
7. Shubh Grah in 1,4,5,7,9,10 and Papa Grah in 3,11,6.
8. After Menses start, day No. 6,8,10,12,14,16 for male progeny.
9. After Menses start, day No. 5,7,9,15 for female progeny.
10. Dhrshti of Jup Mar or Sun on Lagna.

Avoid 

1. Sun light time = [Sunrise - 3 hours] to [Sunset + 3 hours].
2. Sunday, Tuesday, Wednesday and Saturday. 
3. Tithi = 4,6,8,9,14,15,30.
4. Nak = Mula, Bharani, Revati, Magha.
5. Yoga = Mahapath, Vyatipath and Vaighrthi.
6. Eclipse day. 
7. Earthquake day.
8. Meteor Shower day.
9. Animals Howling day.
10. Father's or mother's Shradh day. 
11. 4 days from start of Menses. 
12. Sankranti day.

The following is from Jataka Parijatakam III.18:

Pariharams in case of dealyed child birth or other problems: